Cleanup Todo example

Clarity tweaks for the Todo example:
* The `initialize` logic in the Todo model is redundant to the way `defaults` works. In its present form it's fairly confusing IMO, and there are better examples of how to use `initialize` elsewhere in the app.
* `where` was added back in underscore 1.4.0 for simple cases of `filter` such as this one.
* `Collection#comparator` can take a string for simple attribute sort.