Remove TodoController to mark completed/incompleted

As of Ember 1.8.1 (possible 1.8), it appears you no longer need to setup the TodoController in order to mark an item complete or incomplete. Once the input checkbox is tied to `todo.isCompleted`, marking an item complete or incomplete just works.

The fact that Ember makes this work automatically should probably be discussed, but I don't feel that I have enough Ember knowledge to write that.

Additionally, the links to the jsbin live preview and the changes in `diff` format still need to be updated before this branch is merged.