Added MultiDict.getone to mimic other multi-dict APIs (Django)
Added MultiDict.getone to mimic other multi-dict APIs (Django)

This fixes #179. Thanks Robert Rollins

hkmshb/bottle,mefyl/bottle,taisa007/bottle-ja,CentricWebEstate/bottle,Kjuly/bottle,kevinmel2000/bottle,samstav/bottle,enzzc/bottle,SamuelMarks/bottle,mrdon/bottle,ericfrederich/bottle,ifduyue/bottle,ListFranz/bottle,kevintaw/bottle,khirayama/bottle,myzhan/bottle,khirayama/bottle,prologic/bottle,AWFU/bottle,defnull/bottle,prologic/bottle,kevinmel2000/bottle,StdioA/bottle,hkmshb/bottle,ifduyue/bottle,fkazimierczak/bottle,Inndy/bottle,ListFranz/bottle,xsank/bottle,myzhan/bottle,Inndy/bottle,adampresley/bottle,wangjun/bottle,hayate/bottle,oz123/bottle,taisa007/bottle-ja,samstav/bottle,wangjun/bottle,AWFU/bottle,defnull/bottle,hayate/bottle,enzzc/bottle,SmithSamuelM/bottle,mefyl/bottle,bottlepy/bottle,Kjuly/bottle,oz123/bottle,catroot/bottle,kevintaw/bottle,StdioA/bottle,mrdon/bottle,gaotongfei/bottle,CentricWebEstate/bottle,fkazimierczak/bottle,SamuelMarks/bottle,bottlepy/bottle,catroot/bottle,ericfrederich/bottle,adampresley/bottle,SmithSamuelM/bottle,gaotongfei/bottle