Add class to page based on topic category

I sat down and did it, and it was pretty quick: https://meta.discourse.org/t/how-to-use-category-color-as-background-color/15151?u=riking

![image](https://cloud.githubusercontent.com/assets/627891/2850037/fbb39cd0-d0f0-11e3-9b0d-44d0d489ba10.png)