Support the possibility to use different executors for MBassador

[Dudeplayz]

If the application runs under vert.x (sockjs) and an event is dispatched async to an ui which updates something then the session crashes because everything which updates the ui needs to run in an vert.x context to allow sockjs to run properly. MBassador allows to set a different dispatcher so it could be useful to allow to set vert.x executors at that point.