This [StackOverflow answer](http://stackoverflow.com/questions/11976393/get-github-username-by-id) claims that we should use username. It's not authoritative, but it seems sensible since just about everything you refer to at GitHub uses the username. If we're worried about people changing their username (which is in a place in GitHub with the warning "the may have unintended side effects" or something like that), then we should store both. I'd be inclined to just store the username.

In the unlikely event that someone changes their GitHub username and wants to assert ownership over their extensions, we can deal with it then...