Feature code highlight

Switched all of the code in the documentation on the website to use [Google's Code Prettify](https://code.google.com/p/google-code-prettify/) code highlighting. I always found the code examples in the NUnit docs to be hard to read. They now look like this, although the formatting can be easily changed to [any of the supported themes](http://google-code-prettify.googlecode.com/svn/trunk/styles/index.html).

![2014-10-28 00_23_57-nunit - identityasserts](https://cloud.githubusercontent.com/assets/493828/4803608/a487987e-5e5a-11e4-9576-846cafe826cc.png)

While I was cleaning up the docs, I deleted old ~ backup files and the footers in the docs directory so the whole site uses the same one. The only difference was the date.