Added `icon` helper.

Adds a level of abstraction to the existing `glyphicon` helper, allowing
users to select a different vector icons library. For instance, it is
now possible to display Font Awesome icons by passing the library option:

    <%= icon :heart, library: :font_awesome %>

Note that, since Font Awesome CSS is not loaded by default with Bootstrap
CSS, it's up to the user to make sure the CSS is loaded in the same app.
Add `font_awesome_css` helper in CdnHelper

Since the `icon` helper accepts `library: :font_awesome`, and the
Font Awesome CSS is not loaded by default by Bootstrap CSS, it is
helpful to have an additional CDN helper, so people who want to
include Font Awesome icons can simply write:

```rhtml
<%= stylesheet_link_tag font_awesome_css %>
<%= icon :heart, library: :font_awesome %>
```