Added masking to cost function
Added masking to cost function

When dealing with sequences of different lenghts, this OPTIONALLY
fixes cost function bias to the largest sequences.

bottler/keras,zhmz90/keras,chenych11/keras,keskarnitish/keras,gamer13/keras,jimgoo/keras,JasonTam/keras,nehz/keras,johmathe/keras,meanmee/keras,eulerreich/keras,nt/keras,sjuvekar/keras,harshhemani/keras,nebw/keras,cvfish/keras,jalexvig/keras,iScienceLuvr/keras,stephenbalaban/keras,amy12xx/keras,vseledkin/keras,iamtrask/keras,yingzha/keras,DeepGnosis/keras,printedheart/keras,LIBOTAO/keras,keras-team/keras,brainwater/keras,EderSantana/keras,kemaswill/keras,Yingmin-Li/keras,bboalimoe/keras,hhaoyan/keras,rodrigob/keras,keras-team/keras,imcomking/Convolutional-GRU-keras-extension-,ekamioka/keras,Smerity/keras,pthaike/keras,zxsted/keras,3dconv/keras,why11002526/keras,llcao/keras,ashhher3/keras,wxs/keras,xiaoda99/keras,xurantju/keras,pjadzinsky/keras,abayowbo/keras,rudaoshi/keras,DLlearn/keras,kuza55/keras,relh/keras,ledbetdr/keras,saurav111/keras,florentchandelier/keras,Aureliu/keras,dxj19831029/keras,dolaameng/keras,jasonyaw/keras,zxytim/keras,fmacias64/keras,danielforsyth/keras,daviddiazvico/keras,untom/keras,nzer0/keras,MagicSen/keras