ArrayController learned itemControllerClass.

```javascript
App.ItemController =
  Em.ObjectController.extend();

var arrayController = Em.ArrayController.create({
  itemControllerClass: 'ItemController',
  content: Em.A([0, 1])
});

controller.objectAtContent(0);               // an instance of ItemController
controller.objectAtContent(0).get('content') // 0
```

This partially addresses #1637, but makes no attempt at supporting reusable
views.