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

It looks like this:

![screenshot from 2014-01-22 18 05 18](https://f.cloud.github.com/assets/1391051/1976868/4882c0f8-8390-11e3-916c-0c3f88aecdde.png)
![screenshot from 2014-01-22 18 05 35](https://f.cloud.github.com/assets/1391051/1976869/48b83ff8-8390-11e3-8ba9-3f85d7e9191f.png)