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:

```javascript
App.Model = Ember.Model.extend();

App.Model.reopenClass({
  collectionKey: function() {
    return this.singularName();
  }.property()
});

App.Post = App.Model.extend();
```