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 %>
```
Add surprise link to README
Merge pull request #16 from Fullscreen/icon-helper

Icon helper
Add partial support for Middleman

Ideally, Bh would not only work with Rails, but with any Ruby framework
that handle views, like Middleman and Padrino.

As of now, some helpers already work with Middleman (Cdn, Icon, Glyphicon),
while most of them do not, due to the fact that Padrino and ActionView implement
the same helpers in different ways (e.g., `link_to`, `form_for`).

This commit adds support to Middleman, although for now only the helpers
above will work. Once all helpers are supported, Middleman will be added
to the README and documentation.

This commit also adds a spec/dummy site running Middleman, to make it easier
for developers to see how the HTML generated by the helpers looks like in a
browser. Once again, this site only includes a few helpers for now, but the
plan is to include all of them.
Merge pull request #17 from Fullscreen/middleman-support

Add partial support for Middleman
Bugfix: Removed control class from file_field