New theme

I'm considering changing the theme of the docs. You can see a list of built-in themes here: http://sphinx-doc.org/theming.html#builtin-themes

I chose _nature_ because the others were uglier than our current theme, or had issues with certain things not working (e.g. hiding "show source" button, hiding index, etc.) The nature theme also makes sections easier to read because the headers are more distinct.

The default styled links were difficult to see in my opinion, so I added some custom colouring to make them stand out a bit more.

Here are screenshots of how it looks:

![](http://i.imgur.com/OBeXaAs.png)
![](http://i.imgur.com/SG2hu4s.png)

@Freeseer let me know what you think. If most people prefer the current theme I won't merge this PR.