Update github dependencies

git://github.com/github/github-flavored-markdown.git is no more.  GFMarkdownEditor
was using that repo for its stylesheets, which are no longer available from github.
Grabbed documentation.css and pygments.css from GFMarkdownEditor demo site, but
github has since updated their styles replaced them with the current github
stylesheets.
Completely replace legacy github stylesheets with current css files