Use Ember.get to get collectionKey and rootKey

This change will allow setting collectionKey and rootKey to a computed
property in a superclass of models, for example:

    App.Model = Ember.Model.extend();

    App.Model.reopenClass({
      collectionKey: function() {
        return this.singularName();
      }.property()
    });

    App.Post = App.Model.extend();