Clean up skeleton and module import stuff

This isn't for merging, just to be used as a basis for discussion. Let me know if you know of a better way.
No longer creating the app in this module. We should just define its behavior with `extend()` just like we do for all other Ember classes. The app gets created in `blogger-main.js` instead, when everything is ready, thus dropping the need for `deferReadiness()`/`advanceReadiness()`.

The important piece here is how we require `modules`, which is at this point just a hash that looks like this:

```javascript
{
  Router: <subclass of Ember.Router>,
  PostController: <subclass of Ember.ObjectController>,
  ...
}
```

Those will get "attached" to the `Ember.Application` once you call `App['default'].create()` in `blogger-main.js`.