[FEATURE computed-controller] Add Ember.computed.controller

This is an idea for working with nested array controllers and for sharing array/item controllers between multiple views.

Usage:

```javascript
property: Ember.computed.controller('controllerName','path.to.content')
```

Here is a demo in action: http://jsbin.com/ULuqIPe/1/edit

And here are the docs:

    A computed property that creates a controller. This computed property should
    only be used on a controller. The controller class is specified by
    `controllerName` and must be explicitly defined. The content of the
    controller must be is specified by the `contentPath` parameter. The generated
    controller has access to the parent controller through its `parentController`
    property. Any actions emitted from the generated controller will pass through
    the parent controller.

    ```javascript
    App.PostController = Ember.ObjectController.extend({
      comments: Ember.computed.controller('comments', 'model.comments');
    });

    // Must be defined
    App.CommentsController = Ember.ArrayController.extend();
    ```

    @method computed.controller
    @for Ember
    @param {String} controllerName
    @param {String} contentPath
    @return {Ember.ComputedProperty} computed property which creates a
    controller with content from `contentPath`.