Use font awesome for TOC anchor links

This is a work in progress branch for replacing the image based anchor with [font awesome](http://fortawesome.github.io/Font-Awesome/). GitHub [moved away from image based icons](https://github.com/blog/1106-say-hello-to-octicons) in 2012. Font Awesome is an easy way to achieve size independent anchor links.

This one line change breaks a bunch of tests. In addition, a separate pull request is necessary for gollum/gollum to update the template.css.