Add Github buttons and extend ribbon to home page.

## Github buttons

![gh-buttons](http://f.cl.ly/items/20273P1d2v1Q1Y3b2b1V/Screen%20Shot%202013-06-03%20at%204.50.03%20PM.png)


## Extend Ribbon (no images!)

![extend](http://f.cl.ly/items/1c412h2R3T1I3e1F0c3l/Screen%20Shot%202013-06-03%20at%204.49.52%20PM.png)

@ericf Do I need to add Github Buttons copyright to our LICENSE.md or something? It's (c) Apache 2.0.