WIP: Reopened the `Ember.View` class to add `registerHelper`

This is a a convenience function for creating handlebars helpers from view classes. We've had it in our app for a long time. I thought about submitting this a while ago, but I was under the impression that arbitrarily creating handlebars helpers would be discouraged. However, the addition of `{{input}}` and `{{textarea}}` made me think this might be worth discussing.

Here's the basic idea:
```javascript
  App.UserControlView = Ember.View.extend({
    templateName: 'user-control',
    user: null
  }).helper('userControl')
```

```handlebars
  {{userControl userBinding="createdBy"}}
```

Clearly this is a WIP as there are no tests and crappy docs. I wanted to submit this idea to get general feedback. If others like this idea I'd be happy to finish it off and optionally convert `{{input}}` and `{{textarea}}` to follow this pattern.