Backbone sync for Firebase model

Added Backbone.ModelFirebase, which allows for explicit separation between local and remote data when working with individual models. Documentation file has not been updated.

Usage:
``` JavaScript
var todoID = ''; //some ID
var ToDo = Backbone.Model.extend({
  //some things
});
var todoModel = new ToDo({text: 'text'}, {
  firebase: new Backbone.ModelFirebase("https://<your-namespace>.firebaseio.com/" + todoID)
});
```