Add jQuery UI accordion for API Docs

Adds a jQuery accordion for the growing API docs...! 

- added `#api-accordion` wrapper
- calculate total files in a group (hack-ish to do it in Liquid, but I didn't see a good place to put it in the yaml!)
- added jquery-ui in docs_scripts.html
- initialize it in docs.js


Does that look ok? I'm not a designer and in fact I'm incapable of making beautiful things :cry:

![accordion](https://cloud.githubusercontent.com/assets/2231765/2691715/87752b0a-c389-11e3-8891-f4298820a403.gif)