Upgrade to Font Awesome v4.2

As explained in the [official website](http://fortawesome.github.io/Font-Awesome/3.2.1/), is no longer supported from a long time.

Upgrade to Font Awesome allow us to keep up to date with minor realeased and get regulary benefit of new icons.

I made this integration with two bunches of sed and some manual check and corrections.

Gists of sed subs files:
 * Haml templates: https://gist.github.com/Soullivaneuh/31e252aae10c6f756526
 * Other files: https://gist.github.com/Soullivaneuh/391c04c73a8b0042cd95

All integration was made only on `app` folder, is this correct ?

I can rebase and re-integrate it if other PR are merge before this one.

Thanks for your return.