Ember.Handlebars.ViewHelper: extract viewClass lookup

We have been using CommonJS to clean up the structure of our code. This works great for JavaScript code:

```javascript
var MyClass = Ember.Object.extend( require('app/userSettingsClient', {
  toString: function() {
    return this.get('userSettings.displayName');
  }
});
```

Unfortunately, it doesn't work for Handlebars view names:

```handlebars
{{view "i/cannotUse/requireHereView"}}
```

I had hoped to be able to swap out `Ember.lookup`, but that needs to be an object, and `require` is a function. The code for actually looking up the view class was mixed in with the rest of the `{{view}}` helper code, but simply extracting it gives us the ability to override it easily.