[cookbook] Added cookbook for notifying users of updated application versions

Apologies for this @stefanpenner. I got busy, forgot about it and now haven't managed to do any ember dev since (new job / new baby etc). Your last comment against this mentioned using the router over the controller. If I could get a pointer to how this could be implemented in a more correct / idiomatic way i'd gladly get this one finished. I'm just a bit out of touch with Ember at the moment.