Model once

[davideast]

This PR adds the feature of one-time sync vs realtime sync via the autoSync property.

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

Resolves #74, #84, #85, #86, #89

[coveralls]

Coverage increased (+3.65%) when pulling 4446cf6 on model-once into d9aeb0e on 0.4-release.