Model once

This PR adds the feature of one-time sync vs realtime sync via the `autoSync` property.

```javascript
var Todo = Backbone.Firebase.Model.extend({
  urlRoot: 'https://webapi.firebaseio.com/todos',
  autoSync: false
});

var todo = new Todo({
  id: 1
});

todo.set('name', 'David'); // not persisted
todo.save(); // is persisted
```