Simplify the loader to not depend on a map of possible
types and folder name tuples.

Also.

given:
```javascript
// app/models/post.js
var User = Ember.Object.extend({
});

export default User;
```

The default resolver will be able to fetch the model
factory via the module system.
 
```javascript
container.lookupFactory('model:post') // => <a post factory>
```