Allow for the initial router to be resolved

This PR allows for a custom router to be resolved, before this was not possible.

* [ ] confirm this isn't considered truely breaking
* [ ] verify this fully fixes the EAK issue

Caveat: Although un-document behaviour, breaks some undocumented behaviour.

Current we support setting the Router immediately after `Application#create`

```javascript
App = Ember.Application.create()
App.Router = CustomRouter;
```

This does have the side effect of not setting the namespace on the router.

This PR, makes (if setting a custom routerClass) a requirement at creation time.

```javascript
App = Ember.Application.create({
  Router: CustomRouter
});
```