Add Ember.PromiseProxyMixin

  A low level mixin making ObjectProxy, ObjectController or ArrayController's promise aware.

  ```javascript
  var ObjectPromiseController = Ember.ObjectController.extend(Ember.PromiseProxyMixin);

  var controller = ObjectController.create({
    promise: $.getJSON('/some/remote/data.json')
  });

  controller.then(function(json){
     // the json
  }, function(reason) {
     // the reason why you have no json
  });
  ```

  the controller has bindable attributes which
  track the promises life cycle

  ```javascript
  controller.get('isLoading')   //=> true
  controller.get('isResolved')  //=> false
  controller.get('isRejected')  //=> false
  controller.get('isFulfilled') //=> false
  ```

  When the the $.getJSON completes, and the promise is fulfilled
  with json, the life cycle attributes will update accordingly.

  ```javascript
  controller.get('isLoading')   //=> false
  controller.get('isResolved')  //=> true
  controller.get('isRejected')  //=> false
  controller.get('isFulfilled') //=> true
  ```

  As the controller is an ObjectController, and the json now its content,
  all the json properties will be available directly from the controller.

  ```javascript
  // Assuming the following json:
  {
    firstName: 'Stefan',
    lastName: 'Penner'
  }

  // both properties will accessible on the controller
  controller.get('firstName') //=> 'Stefan'
  controller.get('lastName')  //=> 'Penner'
  ```

  If the controller is backing a template, the attributes are 
  bindable from within that template
  ```handlebars
    {{#if isLoading}}
      loading...
    {{else}}
      firstName: {{firstName}}
      lastName: {{lastName}}
    {{/if}}
  ```