Added widget section and flickr widget in styleguide

Say hello to our new "Widgets" section in the styleguide! 

The first one: Flickr widget. Uses image cards as main component to render the photos. 

![screen shot 2014-06-26 at 12 00 22](https://cloud.githubusercontent.com/assets/1152640/3397111/c5195044-fd18-11e3-9ca3-544d1979f6ba.png)

It would make more sense for this file to rather be named `flickr.html.haml` to match the component name and remove the duplication of `widget`.