Icon helper

* Created `IconHelper`
* Removed `FontAwesome` helper
* Removed `Glyphicon` helper
Let's not completely erase this helper!

I still like it, and it's good for backwards compatibility. It should simply require the new helper and then have

```ruby
 def glyphicon(name = nil, options = {})
  icon name, options.merge(library: :glyphicons)
end
```