Reimplemented App.router as per documentation

I know the router API is in flux, and the documentation probably hasn't been updated for a while, however I've added this back in as it is incredibly useful during development to be able to access the router in the console.

This PR makes `App.router` available as per documentation and the old router. This simply sets App.router to the instance of the first instantiated router, this is practice might not be the desired outcome and I'm sure @tomdale can weigh in on this.