Interactive bubble tree for exploring repositories

Had a go at a fun visualisation of our repos; still WIP, but opening PR for comment. Check out and open index.html to have a play.

![Screen Shot 2013-04-24 at 10 04 04](https://f.cloud.github.com/assets/3565/418980/206d6572-acbe-11e2-8de4-acbf82644577.png)

The code that generates the JSON from github is still just on my machine - haven't decided where to put it yet, or how to keep the vis up to date, so there's still plenty to do.