[FEATURE ember-routing-didTransition-hook] Add a `didTransition` hook to the router.

For use in analytics and such, you may listen to this hook.

```JavaScript
Ember.Application.initializer({
  name: 'analytics',
  initialize: function(container, application){
    var router = container.lookup('router:main');
    router.on('didTransition', function(){
      _gaq.push(['_trackPageview', router.get('url')]);
    });
  }
});
```

Seems good for analytics, though perhaps you could just observe `url` directly? I imagine there are other use-cases.