added tests for stddev
added tests for stddev

Scapogo/zipline,alphaBenj/zipline,chrjxj/zipline,grundgruen/zipline,MonoCloud/zipline,umuzungu/zipline,jimgoo/zipline-fork,florentchandelier/zipline,otmaneJai/Zipline,bartosh/zipline,humdings/zipline,YuepengGuo/zipline,wubr2000/zipline,erikness/AlephOne,jordancheah/zipline,ronalcc/zipline,alphaBenj/zipline,gwulfs/zipline,keir-rex/zipline,semio/zipline,aajtodd/zipline,quantopian/zipline,enigmampc/catalyst,enigmampc/catalyst,sketchytechky/zipline,CDSFinance/zipline,zhoulingjun/zipline,humdings/zipline,keir-rex/zipline,gwulfs/zipline,grundgruen/zipline,sketchytechky/zipline,davidastephens/zipline,wilsonkichoi/zipline,dkushner/zipline,kmather73/zipline,bartosh/zipline,umuzungu/zipline,semio/zipline,michaeljohnbennett/zipline,dhruvparamhans/zipline,morrisonwudi/zipline,wubr2000/zipline,kmather73/zipline,dmitriz/zipline,magne-max/zipline-ja,mattcaldwell/zipline,iamkingmaker/zipline,CDSFinance/zipline,euri10/zipline,davidastephens/zipline,dkushner/zipline,nborggren/zipline,dmitriz/zipline,magne-max/zipline-ja,iamkingmaker/zipline,CarterBain/AlephNull,CarterBain/AlephNull,stkubr/zipline,nborggren/zipline,florentchandelier/zipline,quantopian/zipline,joequant/zipline,aajtodd/zipline,otmaneJai/Zipline,cmorgan/zipline,mattcaldwell/zipline,MonoCloud/zipline,DVegaCapital/zipline,jimgoo/zipline-fork,morrisonwudi/zipline,ChinaQuants/zipline,euri10/zipline,chrjxj/zipline,ronalcc/zipline,YuepengGuo/zipline,DVegaCapital/zipline,StratsOn/zipline,AlirezaShahabi/zipline,jordancheah/zipline,zhoulingjun/zipline,joequant/zipline,erikness/AlephOne,michaeljohnbennett/zipline,Scapogo/zipline,AlirezaShahabi/zipline,stkubr/zipline,ChinaQuants/zipline,cmorgan/zipline,dhruvparamhans/zipline,StratsOn/zipline,wilsonkichoi/zipline