Document Set Proxies and Set Indexes

Adds docs for:

- SetProxy
- SetSort
- SetIndex
- UniqueSetIndex

as children of Batman.Set:

![screen shot 2014-02-05 at 3 13 39 pm](https://f.cloud.github.com/assets/2231765/2093576/3d9ebe40-8ebb-11e3-93ca-98275526d896.png)

Whaddya think?