Apply model defaults

I'm still new to Firebase, so I'd love some eyes on this. Basically, I noticed that when setting a model's `defaults` property, it would get overwritten by the first server-side sync.

This commit defers setting the default attributes until after the first sync, making sure not to override what's in Firebase.


I also crushed some whitespace. If that hurts your eyes, you can [view this diff without it](https://github.com/firebase/backfire/pull/50/files?w=1).