Notifiable app

@FWKamil I spent some time extracting a common Rails App (with webpages etc) which we could use for *multiple* Apps (with different keys etc). 

Everything pointed to needing an "App" class inside the project - which was fine. However, the device token and the notification then needed to be connected to this. 

It felt very much like this should be part of the core gem. I think it would be sensible to add it here then refactor out in the future if we need to.

Should still be straightforward enough to configure for "single App" use. 

Let me know what you think.

Matt