New component .btn-link, to use buttons styled as links

Instead of using `<a>` elements as script triggers, we should use the semantic element `<button>` but styled as a link.

![btn-link preview](https://f.cloud.github.com/assets/188748/1067941/bbcc8466-13df-11e3-84bc-18af71559db6.png)

Inspired by Twitter Bootstrap http://getbootstrap.com/css/#buttons