Memory usage upgrade
Memory usage upgrade

autorelease the bitmap context for clearing cache when memory warnings
cochat/SDWebImage,Insfgg99x/SDWebImage,dpettigrew/SDWebImage,dreampiggy/SDWebImage,awkward/SDWebImage,buleSky111/SDWebImage,HannaLu/SDWebImage,bpoplauschi/SDWebImage,iheartradio/SDWebImage,heroims/SDWebImage,mattjgalloway/SDWebImage,awkward/SDWebImage,smule/SDWebImage,tokorom/SDWebImage,zhang-yawei/SDWebImage,iosnatarajan/SDWebImage,odyth/SDWebImage,altrooz/SDWebImage,smyrgl/SDWebImage,klundberg/SDWebImage,amyhametov/SDWebImage,ruaho/SDWebImage,twobitlabs/SDWebImage,ttgb/SDWebImage,ikesyo/SDWebImage,dreampiggy/SDWebImage,rs/SDWebImage,geoffmacd/SDWebImage,tualatrix/SDWebImage,yunzhongyoulong/SDWebImage,iMinichrispy/SDWebImage,Kekanto/SDWebImage,awkward/SDWebImage,Frntbac/SDWebImage,zllbird/SDWebImage,dreampiggy/SDWebImage