allow to use files/ on gists
allow to use files/ on gists

closes gh-155

franblas/beaker-sharing-server,christophelec/nbviewer,twosigma/beaker-sharing-server,bollwyvl/nbviewer,twosigma/beaker-sharing-server,Jay-Oh-eN/nbviewer,Jay-Oh-eN/nbviewer,iamjakob/nbviewer,roxyboy/nbviewer,roxyboy/nbviewer,iamjakob/nbviewer,twosigma/beaker-sharing-server,franblas/beaker-sharing-server,imsparsh/nbviewer,Sage-Bionetworks/nbviewer,imsparsh/nbviewer,AlfiyaZi/nbviewer,christophelec/nbviewer,AlfiyaZi/nbviewer,Sage-Bionetworks/nbviewer,bollwyvl/nbviewer,thomasyu888/nbviewer,franblas/beaker-sharing-server,thomasyu888/nbviewer,franblas/beaker-sharing-server,twosigma/beaker-sharing-server