Make footer scroll with you

If your issue/pull request is very long, the footer gets in the way of the text like so:
![screen shot 2014-04-17 at 4 09 49 pm](https://cloud.githubusercontent.com/assets/2197376/2736379/7470feae-c66c-11e3-8381-4cbe15a8e3aa.png)
By using position:fixed, the footer scrolls with you, making text readable.