Add Font Awesome Cheat Sheet. Closes #31

Depends on cheatset gem >= 1.2.11

Git submodule for Font Awesome should be updated before generating the cheatsheet:

    $ git submodule update
    $ cheatset generate cheatsheets/Font_Awesome.rb

To update cheatsheet for a newer version of Font Awesome, update the submodule repository and switch to another version tag:

```
$ cd resources/Font_Awesome/FontAwesome/
$ git fetch # in submodule dir
$ git checkout <tag for new version>
$ cd <cheatsheets dir>
$ git commit resources/Font_Awesome/FontAwesome/
```

And re-generate the cheatsheet.