Removed Bacbone.sync override

Prior to the 0.4 release a new Backbone.Mode could be created with a `firebase` attribute.

```javascript
var Todo = Backbone.Model.extend({
   firebase: 'https://<your-firebase>.firebaseio.com/'
});
```
This behavior is being removed in the 0.4 release. The reasoning behind this decision was due to the confusion of the separate components: `Backbone.Firebase.Model` and `Backbone.Model.extend({ firebase: ''});`

Going forward all of the functionality BackFire provides will be in the `Backbone.Firebase` namespace. There will still be an option that allows for data to **not** automatically be synced. In those cases you will be able to call `sync()`, `save()`, `fetch()`, and etc...

This will leave the core of Backbone untouched, while hopefully reducing confusion.