Feature/r context completions

(Branched off of #187, so we should consider that first before coming here)

This add context, and scope, specific completions for source documents. This is a first stab, but figured it would be good for prompting further discussion.

![screen shot 2014-10-30 at 1 48 26 pm](https://cloud.githubusercontent.com/assets/1976582/4851879/2da9b1ba-6076-11e4-8461-4dcba246a1cb.png)