register `handler_function` to receive `signal_name`.

        Uses class's dbus interface self.IFACE, objects name self.name
        and objects path self.OBJ_PATH to match signal.

        :param str signal_name: The signal name;
                                None(default) matches all names.
        :param function handler_function: The function to be called.

def register_signal_handler(self, signal_name, handler_function):
        """register `handler_function` to receive `signal_name`.

        Uses class's dbus interface self.IFACE, objects name self.name
        and objects path self.OBJ_PATH to match signal.

        :param str signal_name: The signal name;
                                None(default) matches all names.
        :param function handler_function: The function to be called.
        """
        self.bus.add_signal_receiver(signal_wrapper(handler_function),
                                     signal_name=signal_name,
                                     dbus_interface=self.IFACE,
                                     bus_name=self.name,
                                     path=self.OBJ_PATH)