Turn RACCommand into an NSObject
Turn RACCommand into an NSObject

This will make the breaking changes more obvious, and also makes code
more self-documenting (through the use of `executionSignals`).

SuPair/ReactiveCocoa,KuPai32G/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,shuxiashusheng/ReactiveCocoa,ceekayel/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,nikita-leonov/ReactiveCocoa,fhchina/ReactiveCocoa,wpstarnice/ReactiveCocoa,eliperkins/ReactiveCocoa,leichunfeng/ReactiveCocoa,gengjf/ReactiveCocoa,cnbin/ReactiveCocoa,bensonday/ReactiveCocoa,Ricowere/ReactiveCocoa,zhukaixy/ReactiveCocoa,Liquidsoul/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,335g/ReactiveCocoa,vincentiss/ReactiveCocoa,Pingco/ReactiveCocoa,mattpetters/ReactiveCocoa,chieryw/ReactiveCocoa,monkeydbobo/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,takeshineshiro/ReactiveCocoa,CQXfly/ReactiveCocoa,SuPair/ReactiveCocoa,dz1111/ReactiveCocoa,Rupert-RR/ReactiveCocoa,tzongw/ReactiveCocoa,WEIBP/ReactiveCocoa,jeelun/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,AndyZhaoHe/ReactiveCocoa,hilllinux/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,dachaoisme/ReactiveCocoa,add715/ReactiveCocoa,JohnJin007/ReactiveCocoa,qq644531343/ReactiveCocoa,bencochran/ReactiveCocoa,xiaobing2007/ReactiveCocoa,eliperkins/ReactiveCocoa,cogddo/ReactiveCocoa,fanghao085/ReactiveCocoa,walkingsmarts/ReactiveCocoa,zhiwen1024/ReactiveCocoa,335g/ReactiveCocoa,ShawnLeee/ReactiveCocoa,Farteen/ReactiveCocoa,beni55/ReactiveCocoa,towik/ReactiveCocoa,emodeqidao/ReactiveCocoa,LHDsimon/ReactiveCocoa,Pikdays/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,chao95957/ReactiveCocoa,yytong/ReactiveCocoa,SanChain/ReactiveCocoa,liufeigit/ReactiveCocoa,gengjf/ReactiveCocoa,zhenlove/ReactiveCocoa,hoanganh6491/ReactiveCocoa,jrmiddle/ReactiveCocoa,ceekayel/ReactiveCocoa,ioshger0125/ReactiveCocoa,mtxs007/ReactiveCocoa,victorlin/ReactiveCocoa,mattpetters/ReactiveCocoa,walkingsmarts/ReactiveCocoa,wangqi211/ReactiveCocoa,almassapargali/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,yizzuide/ReactiveCocoa,jaylib/ReactiveCocoa,jam891/ReactiveCocoa,wangqi211/ReactiveCocoa,JohnJin007/ReactiveCocoa,tonyli508/ReactiveCocoa,beni55/ReactiveCocoa,zhaoguohui/ReactiveCocoa,taylormoonxu/ReactiveCocoa,Liquidsoul/ReactiveCocoa,Farteen/ReactiveCocoa,tornade0913/ReactiveCocoa,hbucius/ReactiveCocoa,brightcove/ReactiveCocoa,smilypeda/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,Khan/ReactiveCocoa,dachaoisme/ReactiveCocoa,itschaitanya/ReactiveCocoa,richeterre/ReactiveCocoa,stupidfive/ReactiveCocoa,jianwoo/ReactiveCocoa,goodheart/ReactiveCocoa,stevielu/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,OneSmallTree/ReactiveCocoa,pzw224/ReactiveCocoa,yonekawa/ReactiveCocoa,jam891/ReactiveCocoa,esttorhe/ReactiveCocoa,JackLian/ReactiveCocoa,windgo/ReactiveCocoa,stevielu/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,CQXfly/ReactiveCocoa,KJin99/ReactiveCocoa,mxxiv/ReactiveCocoa,yizzuide/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,ztchena/ReactiveCocoa,hilllinux/ReactiveCocoa,isghe/ReactiveCocoa,nickcheng/ReactiveCocoa,natan/ReactiveCocoa,taylormoonxu/ReactiveCocoa,dullgrass/ReactiveCocoa,kiurentu/ReactiveCocoa,swizzlr/ReactiveCocoa,brightcove/ReactiveCocoa,ohwutup/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,ddc391565320/ReactiveCocoa,jaylib/ReactiveCocoa,ohwutup/ReactiveCocoa,Ricowere/ReactiveCocoa,zxq3220122/ReactiveCocoa,jaylib/ReactiveCocoa,sugar2010/ReactiveCocoa,Ethan89/ReactiveCocoa,kevin-zqw/ReactiveCocoa,mxxiv/ReactiveCocoa,mxxiv/ReactiveCocoa,WEIBP/ReactiveCocoa,add715/ReactiveCocoa,richeterre/ReactiveCocoa,esttorhe/ReactiveCocoa,AndyZhaoHe/ReactiveCocoa,xumaolin/ReactiveCocoa,Liquidsoul/ReactiveCocoa,rpowelll/ReactiveCocoa,imkerberos/ReactiveCocoa,eyu1988/ReactiveCocoa,jianwoo/ReactiveCocoa,j364960953/ReactiveCocoa,qq644531343/ReactiveCocoa,libiao88/ReactiveCocoa,Eveian/ReactiveCocoa,nickcheng/ReactiveCocoa,sujeking/ReactiveCocoa,nikita-leonov/ReactiveCocoa,AlanJN/ReactiveCocoa,windgo/ReactiveCocoa,emodeqidao/ReactiveCocoa,calebd/ReactiveCocoa,hj3938/ReactiveCocoa,shaohung001/ReactiveCocoa,j364960953/ReactiveCocoa,dskatz22/ReactiveCocoa,brasbug/ReactiveCocoa,calebd/ReactiveCocoa,ikesyo/ReactiveCocoa,dachaoisme/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,sujeking/ReactiveCocoa,SanChain/ReactiveCocoa,ailyanlu/ReactiveCocoa,hbucius/ReactiveCocoa,tornade0913/ReactiveCocoa,LHDsimon/ReactiveCocoa,KuPai32G/ReactiveCocoa,towik/ReactiveCocoa,on99/ReactiveCocoa,BrooksWon/ReactiveCocoa,monkeydbobo/ReactiveCocoa,fanghao085/ReactiveCocoa,mtxs007/ReactiveCocoa,yoichitgy/ReactiveCocoa,Eveian/ReactiveCocoa,Khan/ReactiveCocoa,gabemdev/ReactiveCocoa,BrooksWon/ReactiveCocoa,rpowelll/ReactiveCocoa,leelili/ReactiveCocoa,ztchena/ReactiveCocoa,llb1119/test,zhigang1992/ReactiveCocoa,chieryw/ReactiveCocoa,Remitly/ReactiveCocoa,jsslai/ReactiveCocoa,chieryw/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,victorlin/ReactiveCocoa,fhchina/ReactiveCocoa,koamac/ReactiveCocoa,yonekawa/ReactiveCocoa,jeelun/ReactiveCocoa,Khan/ReactiveCocoa,lixar/ReactiveCocoa,Ethan89/ReactiveCocoa,yytong/ReactiveCocoa,ikesyo/ReactiveCocoa,andersio/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,shuxiashusheng/ReactiveCocoa,yytong/ReactiveCocoa,imkerberos/ReactiveCocoa,brasbug/ReactiveCocoa,howandhao/ReactiveCocoa,kiurentu/ReactiveCocoa,kevin-zqw/ReactiveCocoa,smilypeda/ReactiveCocoa,Remitly/ReactiveCocoa,Adlai-Holler/ReactiveCocoa,dz1111/ReactiveCocoa,alvinvarghese/ReactiveCocoa,yizzuide/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,zxq3220122/ReactiveCocoa,sandyway/ReactiveCocoa,sandyway/ReactiveCocoa,imkerberos/ReactiveCocoa,koamac/ReactiveCocoa,ericzhou2008/ReactiveCocoa,takeshineshiro/ReactiveCocoa,dullgrass/ReactiveCocoa,vincentiss/ReactiveCocoa,hbucius/ReactiveCocoa,ioshger0125/ReactiveCocoa,luerhouhou/ReactiveCocoa,xulibao/ReactiveCocoa,chao95957/ReactiveCocoa,yoichitgy/ReactiveCocoa,isghe/ReactiveCocoa,howandhao/ReactiveCocoa,shaohung001/ReactiveCocoa,AndyZhaoHe/ReactiveCocoa,leelili/ReactiveCocoa,cogddo/ReactiveCocoa,zxq3220122/ReactiveCocoa,xiaobing2007/ReactiveCocoa,qq644531343/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,zzzworm/ReactiveCocoa,WEIBP/ReactiveCocoa,natestedman/ReactiveCocoa,dz1111/ReactiveCocoa,200895045/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,alvinvarghese/ReactiveCocoa,valleyman86/ReactiveCocoa,stevielu/ReactiveCocoa,gengjf/ReactiveCocoa,JohnJin007/ReactiveCocoa,terry408911/ReactiveCocoa,tonyli508/ReactiveCocoa,200895045/ReactiveCocoa,SanChain/ReactiveCocoa,jackywpy/ReactiveCocoa,almassapargali/ReactiveCocoa,terry408911/ReactiveCocoa,nickcheng/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,icepy/ReactiveCocoa,nickcheng/ReactiveCocoa,OneSmallTree/ReactiveCocoa,ShawnLeee/ReactiveCocoa,Ray0218/ReactiveCocoa,Ray0218/ReactiveCocoa,on99/ReactiveCocoa,JackLian/ReactiveCocoa,paulyoung/ReactiveCocoa,KuPai32G/ReactiveCocoa,taylormoonxu/ReactiveCocoa,dskatz22/ReactiveCocoa,KJin99/ReactiveCocoa,cstars135/ReactiveCocoa,ailyanlu/ReactiveCocoa,DreamHill/ReactiveCocoa,valleyman86/ReactiveCocoa,victorlin/ReactiveCocoa,200895045/ReactiveCocoa,smilypeda/ReactiveCocoa,huiping192/ReactiveCocoa,SmartEncounter/ReactiveCocoa,llb1119/test,zhukaixy/ReactiveCocoa,xiaoliyang/ReactiveCocoa,clg0118/ReactiveCocoa,Juraldinio/ReactiveCocoa,leichunfeng/ReactiveCocoa,tipbit/ReactiveCocoa,SuPair/ReactiveCocoa,loupman/ReactiveCocoa,ddc391565320/ReactiveCocoa,sdhzwm/ReactiveCocoa,pzw224/ReactiveCocoa,isghe/ReactiveCocoa,zhigang1992/ReactiveCocoa,howandhao/ReactiveCocoa,Ricowere/ReactiveCocoa,libiao88/ReactiveCocoa,FelixYin66/ReactiveCocoa,cstars135/ReactiveCocoa,jsslai/ReactiveCocoa,swizzlr/ReactiveCocoa,mattpetters/ReactiveCocoa,jam891/ReactiveCocoa,clg0118/ReactiveCocoa,j364960953/ReactiveCocoa,xulibao/ReactiveCocoa,FelixYin66/ReactiveCocoa,huiping192/ReactiveCocoa,almassapargali/ReactiveCocoa,llb1119/test,zzzworm/ReactiveCocoa,kaylio/ReactiveCocoa,DreamHill/ReactiveCocoa,bscarano/ReactiveCocoa,ailyanlu/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,windgo/ReactiveCocoa,xumaolin/ReactiveCocoa,luerhouhou/ReactiveCocoa,ericzhou2008/ReactiveCocoa,yonekawa/ReactiveCocoa,clg0118/ReactiveCocoa,Adlai-Holler/ReactiveCocoa,xiaoliyang/ReactiveCocoa,paulyoung/ReactiveCocoa,leelili/ReactiveCocoa,walkingsmarts/ReactiveCocoa,shaohung001/ReactiveCocoa,zhiwen1024/ReactiveCocoa,ztchena/ReactiveCocoa,ioshger0125/ReactiveCocoa,ShawnLeee/ReactiveCocoa,chao95957/ReactiveCocoa,juliangrosshauser/ReactiveCocoa,itschaitanya/ReactiveCocoa,cnbin/ReactiveCocoa,tonyarnold/ReactiveCocoa,cnbin/ReactiveCocoa,buildo/ReactiveCocoa,BrooksWon/ReactiveCocoa,andersio/ReactiveCocoa,zzqiltw/ReactiveCocoa,Remitly/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,calebd/ReactiveCocoa,liufeigit/ReactiveCocoa,koamac/ReactiveCocoa,eliperkins/ReactiveCocoa,Pingco/ReactiveCocoa,huiping192/ReactiveCocoa,tonyarnold/ReactiveCocoa,hoanganh6491/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,jackywpy/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,emodeqidao/ReactiveCocoa,jackywpy/ReactiveCocoa,Pingco/ReactiveCocoa,stupidfive/ReactiveCocoa,natan/ReactiveCocoa,ericzhou2008/ReactiveCocoa,FelixYin66/ReactiveCocoa,goodheart/ReactiveCocoa,BlessNeo/ReactiveCocoa,zhiwen1024/ReactiveCocoa,hilllinux/ReactiveCocoa,eyu1988/ReactiveCocoa,kaylio/ReactiveCocoa,kaylio/ReactiveCocoa,natestedman/ReactiveCocoa,sugar2010/ReactiveCocoa,libiao88/ReactiveCocoa,kevin-zqw/ReactiveCocoa,loupman/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,sdhzwm/ReactiveCocoa,zhigang1992/ReactiveCocoa,goodheart/ReactiveCocoa,yoichitgy/ReactiveCocoa,tornade0913/ReactiveCocoa,tonyarnold/ReactiveCocoa,Farteen/ReactiveCocoa,lixar/ReactiveCocoa,xumaolin/ReactiveCocoa,xiaoliyang/ReactiveCocoa,jrmiddle/ReactiveCocoa,BlessNeo/ReactiveCocoa,valleyman86/ReactiveCocoa,DreamHill/ReactiveCocoa,jsslai/ReactiveCocoa,terry408911/ReactiveCocoa,natan/ReactiveCocoa,tiger8888/ReactiveCocoa,tipbit/ReactiveCocoa,cogddo/ReactiveCocoa,zzqiltw/ReactiveCocoa,lixar/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,fhchina/ReactiveCocoa,Juraldinio/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,Ray0218/ReactiveCocoa,BlessNeo/ReactiveCocoa,bensonday/ReactiveCocoa,longv2go/ReactiveCocoa,longv2go/ReactiveCocoa,itschaitanya/ReactiveCocoa,wangqi211/ReactiveCocoa,SmartEncounter/ReactiveCocoa,Pikdays/ReactiveCocoa,stupidfive/ReactiveCocoa,AllanChen/ReactiveCocoa,hoanganh6491/ReactiveCocoa,sugar2010/ReactiveCocoa,bscarano/ReactiveCocoa,tzongw/ReactiveCocoa,tzongw/ReactiveCocoa,paulyoung/ReactiveCocoa,LHDsimon/ReactiveCocoa,bencochran/ReactiveCocoa,xiaobing2007/ReactiveCocoa,zzqiltw/ReactiveCocoa,JackLian/ReactiveCocoa,mtxs007/ReactiveCocoa,esttorhe/ReactiveCocoa,jeelun/ReactiveCocoa,leichunfeng/ReactiveCocoa,juliangrosshauser/ReactiveCocoa,buildo/ReactiveCocoa,loupman/ReactiveCocoa,zhenlove/ReactiveCocoa,pzw224/ReactiveCocoa,dullgrass/ReactiveCocoa,ceekayel/ReactiveCocoa,Rupert-RR/ReactiveCocoa,vincentiss/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,tiger8888/ReactiveCocoa,kiurentu/ReactiveCocoa,andersio/ReactiveCocoa,brightcove/ReactiveCocoa,icepy/ReactiveCocoa,on99/ReactiveCocoa,add715/ReactiveCocoa,ddc391565320/ReactiveCocoa,AlanJN/ReactiveCocoa,wpstarnice/ReactiveCocoa,xulibao/ReactiveCocoa,sujeking/ReactiveCocoa,KJin99/ReactiveCocoa,wpstarnice/ReactiveCocoa,OneSmallTree/ReactiveCocoa,eyu1988/ReactiveCocoa,buildo/ReactiveCocoa,Carthage/ReactiveCocoa,jianwoo/ReactiveCocoa,icepy/ReactiveCocoa,zzzworm/ReactiveCocoa,jaylib/ReactiveCocoa,Ethan89/ReactiveCocoa,natestedman/ReactiveCocoa,AllanChen/ReactiveCocoa,Eveian/ReactiveCocoa,huiping192/ReactiveCocoa,CQXfly/ReactiveCocoa,alvinvarghese/ReactiveCocoa,beni55/ReactiveCocoa,richeterre/ReactiveCocoa,Rupert-RR/ReactiveCocoa,takeshineshiro/ReactiveCocoa,Carthage/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,sandyway/ReactiveCocoa,juliangrosshauser/ReactiveCocoa,monkeydbobo/ReactiveCocoa,shuxiashusheng/ReactiveCocoa,brasbug/ReactiveCocoa,zhaoguohui/ReactiveCocoa,bscarano/ReactiveCocoa,Carthage/ReactiveCocoa,hj3938/ReactiveCocoa,towik/ReactiveCocoa,nikita-leonov/ReactiveCocoa,Adlai-Holler/ReactiveCocoa,bencochran/ReactiveCocoa,zhukaixy/ReactiveCocoa,AlanJN/ReactiveCocoa,335g/ReactiveCocoa,Pikdays/ReactiveCocoa,rpowelll/ReactiveCocoa,tiger8888/ReactiveCocoa,tonyli508/ReactiveCocoa,jrmiddle/ReactiveCocoa,dskatz22/ReactiveCocoa,Ricowere/ReactiveCocoa,zhenlove/ReactiveCocoa,esttorhe/ReactiveCocoa,gabemdev/ReactiveCocoa,bensonday/ReactiveCocoa,fanghao085/ReactiveCocoa,liufeigit/ReactiveCocoa,hj3938/ReactiveCocoa,longv2go/ReactiveCocoa,Juraldinio/ReactiveCocoa,cstars135/ReactiveCocoa,zhaoguohui/ReactiveCocoa,luerhouhou/ReactiveCocoa,ohwutup/ReactiveCocoa,sdhzwm/ReactiveCocoa