Register an event class on it's name with a method to process it.
        :param event_name: name of the event.
        :param event: class of the event.
        :param method: a method used to process this event.

def register_event(cls, event_name, event, method):
        """
        Register an event class on it's name with a method to process it.
        :param event_name: name of the event.
        :param event: class of the event.
        :param method: a method used to process this event.
        """
        log.info('@Registry.register_event `{}` with subscriber `{}`'
                 .format(event_name, method.__name__))

        if event_name not in cls._events:
            cls._events[event_name] = {}

        if event not in cls._events[event_name]:
            cls._events[event_name][event] = []

        cls._events[event_name][event].append(method)