Make tests pass with Ember Data.

Here's an attempt to make jshint and the model tests pass on the Ember Data branch. Two tests still fail:

* "a Todo begins with completed set to false"
* "destroying a Todo proxies to the store" – guessing that this needs some commit() calls, but I didn't manage to get it to work

I'm a complete noob with Ember, so this may need some more work. I'm not sure if these tests are still relevant when using Ember Data, or if new ones are needed.