Incorporate font awesome

Another attempt to have Font-Awesome icons available. The problem is that Font-Awesome's css selectos are all less specific than the ones from reveal.js, such that nothing works. Instead of manually editing the css rules as in #917, I added a Sass file that directly references the Sass files from Font-Awesome, such that minimal changes are required when updating to a new version. Font-Awesome is referenced using a git submodule.