Improve the home page by adding more content, GitHub Call-to-action

This pull request does the following:

* Adds a "View on GitHub" button to the header. 
* Adds a ribbon on the home page highlighting the ease of extending Pure. No images FTW.
* Adds some Skin Builder content to the Extend page, so that people looking to write CSS on top of Pure can consider the Skin Builder as an option.

Home page image:

![](http://f.cl.ly/items/2m0R2P3O2a2O1L1z021B/Screen%20Shot%202013-06-03%20at%205.22.14%20PM.png)

Extend Ribbon image:

![](http://f.cl.ly/items/1c412h2R3T1I3e1F0c3l/Screen%20Shot%202013-06-03%20at%204.49.52%20PM.png)