One time syncing for collections

This PR adds the feature to have one time syncing for collections by setting autoSync to false.

```javascript
var Todos = Backbone.Firebase.Collection.extend({
  url: 'https://backbone-firebase.firebaseio-demo.com/todos',
  autoSync: false
});

var model = todo.add({ title: 'One time persisting' }); // adds locally
model.save(); // persists to Firebase
```