Added contributors to header

Thought it would be cool to have the list of contributors to the Puma project in the Engine Yard header like on RailsInstaller site. Uses the `gh_contributors` gem from @mpapis. Check out the screen shot below:

![Screen Shot 2013-03-03 at 12 12 50 AM](https://f.cloud.github.com/assets/169018/213852/13d58310-83c1-11e2-91e5-d71e6b1b3722.png)