Identity next

This PR just updates the branding in the header and footer to match the [recently released](https://github.com/blog/1472-new-github-logo) branding iterations.

Screenshots:
![](https://f.cloud.github.com/assets/296432/372613/5e701788-a364-11e2-819b-70556b7b7369.png)
![](https://f.cloud.github.com/assets/296432/372614/60e0ceea-a364-11e2-8078-9f71297c665c.png)