Updates pairtrade example to use window_length.
Updates pairtrade example to use window_length.

Batch transforms now use window_length instead of days.

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