TodoController unnecessary to mark model complete

As of Ember 1.8.1 (possibly 1.8), it doesn't appear to be necessary to create the TodoController in order to be able to mark todos as complete or incomplete. In this commit, I removed the unnecessary code. It might be better to include a description of what Ember is doing and why the TodoController is no longer required. Although, I don't have the Ember expertise to provide that information at this time.