Overhauled mod selector.

This adds a new global mod selector that improves the visibility of our other mods and ties in with the website design.

![chooser](https://f.cloud.github.com/assets/167819/2494952/bdb62fae-b2de-11e3-89f8-c219261b93b0.png)

This is mainly for feedback/discussion at this stage; it could do with a little more polish before merging.