Add Gittip button to the top

![Screenshot from 2013-04-23 10:17:59](https://f.cloud.github.com/assets/9906/413340/6e8cbec2-abee-11e2-86da-a46a9a9a0551.png)

Turns out Gittip has CORS enabled, so no need for ugly `!important` hacks.