Feature: subscriber registeration with downcasting event type.

Hi,

I've built an event barrier library with EventBus to trigger an action by a composite event.
For example turn on location service if battery ok, online and oauth session is valid. This approach uses
heavily a composite pattern (GoF).

Now I have to write a lot of stub classes to declare onEvent with required argument type.
With ability to point obviously event type of subscriber this can be done in runtime.  
Logic of all my event handlers is the same. So in this case it is redundant to declare one method per event.
It's more convenient just to list base atomic event classes as arguments of a builder method.