Use model.urlRoot as the storeName, when available, before model.url

This fixes #80 where models with the same `urlRoot`
that are intended to be part of the same collection end up in different stor
when the collection attribute is not set on the model.

**Existing apps that rely on this incorrect behavior may break.**

If your app expects models with the same `urlRoot` and differing `url`s to
be in different stores locally, use the following workaround in your model:

      storeName: function() { return this.url() }
Bring changelog up to date