One time syncing for collections

[davideast]

This PR adds the feature to have one time syncing for collections by setting autoSync to false.

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

Resolves #86, #91

[coveralls]

Coverage increased (+2.82%) when pulling f74e243 on collection-o into f34f820 on 0.4-release.