Update favicons to use a centered name

Per #127, updates the site's favicon to make the name of the team more legible.

![18f-center-200](https://cloud.githubusercontent.com/assets/4592/3780604/0ebccde8-198e-11e4-8d78-a2aef6b0a1f2.png)

![screenshot_2014-08-01-10-45-15](https://cloud.githubusercontent.com/assets/4592/3780607/183382ea-198e-11e4-82f2-06aeab8ea43e.png)