Icon helper

* Created `IconHelper`
* Removed `FontAwesome` helper
* Removed `Glyphicon` helper
If I read your `case` statement correctly, it says

```ruby
case options.delete(:library)
  when :font_awesome, 'font-awesome' then :fa
  else options[:library]
  :glyphicon
end
```

So I have a feeling that when the `else` statement is reached, then `:glyphicon`is always returned, rather than `options[:library]`.
Maybe it's just me reading the code on GitHub…