Icon helper

* Created `IconHelper`
* Removed `FontAwesome` helper
* Removed `Glyphicon` helper
Mmmh… I need to take a look at those options! It's a good idea to add them, but I still wouldn't want the code to be too specific to Font Awesome. 

For instance, if someone writes

```ruby
<%= icon :heart, library: :glyphicons, size: 3 %>
```

I wouldn't mind displaying `<span class="glyphicon glyphicon-zoom-in glyphicon-3x"></span>` even if that class does not have a meaning for glyphicon.

At least there would be more consistency in the helper, and less library-specific options.

I'll have to look up those options!