Icon helper

Hello @buren ! This PR stems from #14 – let me tell you what I've changed:

1. I have squashed your commits into one
2. I have temporarily removed the additional options of Font Awesome icons (size, fixed width). I don't mean to remove them forever, I just need more time to think about the option names. Meanwhile, already with this PR, you can still have different sizes by using the `:class` option, such as `<%= icon :heart, library: :font_awesome, class: 'fa-2x' %>`… 
3. I have added a `font_awesome_css` helper to CDN, so it's even easier to use Font Awesome icons.
4. I have cleaned up the documentation and the specs a little bit
5. I have left the Glyphicon helper for now… for backwards-compatibility and I don't mind having two methods in this case.

If it all looks good to you, I'll merge and add the appropriate documentation to http://fullscreen.github.io/bh

Thanks again for the idea and the coding!!