Add NodeReferenceCollection/EntityReferenceCollection Collection Type

It looks like we'll be implementing a basic "hasMany" relationship then (which makes sense). I'd think this makes the most sense to represent with a Model that has a Collection as a property to hold the various nodereference nodes.

As a first version, I think we need to provide a Drupal.Backbone.NodeRefCollection object and instructions for using it to represent nodes that had nodereference fields. I've included some pseudocode for what that might look like below. Our NodeRefCollection will likely also need a toJSON method that provides JSON in the correct form for the Drupal server/save routines.

This all begins to get us down the road of content-type specific parse, initialize and toJSON functions. Ultimately, we can construct these somewhat dynamically using behaviors, and give Developers the option of using custom created models or generated per-type model classes. For the initial iteration, however, I think we need to give developers the tools they need to create Backbone models that can hold complex field types.

NodeWithNodeRefField = Drupal.Backbone.Model.extend({
  initialize: {
    this.set('noderef_field',{"und": new Drupal.Backbone.NodeRefCollection([{...}]?);
  },
  ...
  parse: function(response) {
    this.set('noderef_field', this.get('noderef_field').reset(response.node_ref_values_array);
  },
  ...
});

For reference, see the Backbone docs on Nested Models & Collections, which details this pattern and mentions others, as well as the Backbone Nested plugin, from the Backbone.js GitHub wiki page.