Add cloning constructor to folly::IOBuf
Add cloning constructor to folly::IOBuf

Summary:
``` lang=cpp
folly::IOBuf buf(folly::IOBuf::CLONE, other);
```

is more convenient (and expression-friendly) than

``` lang=cpp
folly::IOBuf buf;
other.cloneInto(buf);
```

Test Plan: used it

Reviewed By: davejwatson@fb.com

Subscribers: net-systems@, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2124605

Signature: t1:2124605:1433372767:c167a36ffdc6d20c740aa57895532e9445766fce

loversInJapan/folly,sonnyhu/folly,KSreeHarsha/folly,wildinto/folly,xzmagic/folly,rklabs/folly,guker/folly,bsampath/folly,tempbottle/folly,xzmagic/folly,leolujuyi/folly,project-zerus/folly,KSreeHarsha/folly,bsampath/folly,bobegir/folly,chjp2046/folly,floxard/folly,Hincoin/folly,rklabs/folly,bsampath/folly,tomhughes/folly,guker/folly,juniway/folly,PPC64/folly,renyinew/folly,SeanRBurton/folly,chjp2046/folly,floxard/folly,clearlylin/folly,PoisonBOx/folly,cole14/folly,bowlofstew/folly,constantine001/folly,bobegir/folly,bikong2/folly,sakishum/folly,Orvid/folly,bobegir/folly,shaobz/folly,CJstar/folly,clearlylin/folly,raphaelamorim/folly,zhiweicai/folly,brunomorishita/folly,juniway/folly,yangjin-unique/folly,chjp2046/folly,Eagle-X/folly,SammyK/folly,nickhen/folly,leolujuyi/folly,mqeizi/folly,cole14/folly,cole14/folly,constantine001/folly,guker/folly,zhiweicai/folly,Hincoin/folly,PoisonBOx/folly,fw1121/folly,fw1121/folly,mqeizi/folly,raphaelamorim/folly,Orvid/folly,charsyam/folly,facebook/folly,tempbottle/folly,Hincoin/folly,CJstar/folly,zhiweicai/folly,reddit/folly,floxard/folly,PPC64/folly,gaoyingie/folly,project-zerus/folly,bikong2/folly,SammyK/folly,tomhughes/folly,renyinew/folly,Orvid/folly,kernelim/folly,xzmagic/folly,theiver9827/folly,constantine001/folly,nickhen/folly,shaobz/folly,SeanRBurton/folly,PPC64/folly,stonegithubs/folly,gavioto/folly,PoisonBOx/folly,arg0/folly,Eagle-X/folly,fw1121/folly,gaoyingie/folly,xzmagic/folly,PoisonBOx/folly,alexst07/folly,chjp2046/folly,bobegir/folly,theiver9827/folly,doctaweeks/folly,chjp2046/folly,upsoft/folly,SeanRBurton/folly,xzmagic/folly,gavioto/folly,KSreeHarsha/folly,floxard/folly,bsampath/folly,colemancda/folly,facebook/folly,facebook/folly,doctaweeks/folly,sakishum/folly,hongliangzhao/folly,upsoft/folly,fw1121/folly,gavioto/folly,brunomorishita/folly,reddit/folly,colemancda/folly,KSreeHarsha/folly,upsoft/folly,gavioto/folly,wildinto/folly,tomhughes/folly,Hincoin/folly,wildinto/folly,facebook/folly,yangjin-unique/folly,Eagle-X/folly,bikong2/folly,CJstar/folly,alexst07/folly,clearlylin/folly,bikong2/folly,wildinto/folly,lifei/folly,bsampath/folly,yangjin-unique/folly,KSreeHarsha/folly,sakishum/folly,doctaweeks/folly,zhiweicai/folly,project-zerus/folly,SeanRBurton/folly,tempbottle/folly,guker/folly,Eagle-X/folly,arg0/folly,kernelim/folly,alexst07/folly,bowlofstew/folly,shaobz/folly,sonnyhu/folly,rklabs/folly,stonegithubs/folly,gaoyingie/folly,colemancda/folly,nickhen/folly,arg0/folly,renyinew/folly,doctaweeks/folly,facebook/folly,leolujuyi/folly,rklabs/folly,reddit/folly,hongliangzhao/folly,sakishum/folly,PoisonBOx/folly,SammyK/folly,theiver9827/folly,tempbottle/folly,nickhen/folly,alexst07/folly,leolujuyi/folly,yangjin-unique/folly,upsoft/folly,Orvid/folly,constantine001/folly,renyinew/folly,clearlylin/folly,raphaelamorim/folly,hongliangzhao/folly,nickhen/folly,stonegithubs/folly,alexst07/folly,loversInJapan/folly,doctaweeks/folly,kernelim/folly,SammyK/folly,lifei/folly,SammyK/folly,mqeizi/folly,loversInJapan/folly,charsyam/folly,constantine001/folly,project-zerus/folly,stonegithubs/folly,renyinew/folly,brunomorishita/folly,mqeizi/folly,mqeizi/folly,cole14/folly,kernelim/folly,bowlofstew/folly,bobegir/folly,gaoyingie/folly,bowlofstew/folly,tomhughes/folly,colemancda/folly,shaobz/folly,fw1121/folly,juniway/folly,Eagle-X/folly,loversInJapan/folly,loversInJapan/folly,brunomorishita/folly,gaoyingie/folly,Orvid/folly,sonnyhu/folly,PPC64/folly,reddit/folly,tomhughes/folly,charsyam/folly,sakishum/folly,gavioto/folly,lifei/folly,kernelim/folly,sonnyhu/folly,juniway/folly,leolujuyi/folly,yangjin-unique/folly,project-zerus/folly,PPC64/folly,raphaelamorim/folly,reddit/folly,zhiweicai/folly,lifei/folly,colemancda/folly,shaobz/folly,floxard/folly,arg0/folly,raphaelamorim/folly,guker/folly,arg0/folly,stonegithubs/folly,theiver9827/folly,cole14/folly,wildinto/folly,lifei/folly,CJstar/folly,bikong2/folly,Hincoin/folly,rklabs/folly,hongliangzhao/folly,sonnyhu/folly,tempbottle/folly,brunomorishita/folly,theiver9827/folly,SeanRBurton/folly,CJstar/folly,juniway/folly,hongliangzhao/folly,charsyam/folly,bowlofstew/folly,upsoft/folly,clearlylin/folly