Navigation simplifcation, homepage style updates

* Icons on the homepage are now larger and more colourful
* Navigation is simplified so that it's less broken up
 * Dropdown navigation elements to directly access headings on other pages is maybe a little overkill - I certainly found them a bit confusing
 * I merged the download and develop pages, as develop was basically only a single paragraph.
* Config change to update the `{{base.url}}` to a non-empty string which will work with local and remote testing.
 * Corresponding link updates through the site.

![image](https://cloud.githubusercontent.com/assets/465550/4285364/e19395a6-3d85-11e4-87c5-eb6696f17080.png)