Start the Engel application by initializing all registered services and starting an Autobahn IOLoop.

        :param on_exit_callback: Callback triggered on application exit

def start(self, on_exit_callback=None):
        """
        Start the Engel application by initializing all registered services and starting an Autobahn IOLoop.

        :param on_exit_callback: Callback triggered on application exit
        """
        # TODO: Support params for services by mapping {servicename: {class,
        # params}}?
        for service in self.services.keys():
            self.services[service] = self.services[service]()

        self.server.start(on_exit_callback)