Add emptyView support.

This commit adds emptyView support, similar to the `Ember.CollectionView`, so the user can interface with the view as they might expect when subclassing. I also removed some unused variables.

Usage:
```javascript
Ember.ListView.extend({
  emptyView: Ember.View.extend({
    template: Ember.Handlebars.compile("no items")
  })
});
```