Fetch users by GitHub ID instead of username

This will also update the username if the user was renamed on GitHub.

Warning:

![no_idea](https://f.cloud.github.com/assets/24421/463475/8de6e596-b566-11e2-8be6-555f3dce9e4b.jpg)

I don't know if this is the right place to do this, but a quick `ag` told me that this service is being sent the entire `owner` payload, so either way it seemed to make sense to do this. If this is the right place to do this, though, we may behave more nicely when users change their username.

I'd be happy to test this out on staging, although I have no idea what component actually creates users. API, when a user logs in?

/cc @rkh