Retheme a little and add a background image

This PR is a few theme tweaks and general tidy up a little:

Changelog:
* Fixed purple colour for active links on navbar
* added a background image
* removed the footer
* Made the download buttons on the home page inverse on hover
* Add a 'Fork Me On GitHub' banner

It looks like this:
![screenshot from 2014-01-22 18 30 21](https://f.cloud.github.com/assets/1391051/1977081/52d061f2-8393-11e3-9547-8204de207e02.png)
![screenshot from 2014-01-22 18 30 15](https://f.cloud.github.com/assets/1391051/1977080/52d044ce-8393-11e3-90ba-36fdc4db180e.png)