[rosserial_python] Use rospy.on_shutdown in SerialClient shutdown

[708yamaguchi]

When I use serial_node.py from roslaunch, we cannot kill serial_node.py by Ctrl-c.

This is because serial_node.py does not catch SystemExit exception caused by sys.exit(0) in txStopRequest.

rosserial/rosserial_python/nodes/serial_node.py

Lines 94 to 109 in c169ae2

	try:
	client = SerialClient(port_name, baud, fix_pyserial_for_test=fix_pyserial_for_test)
	client.run()
	except KeyboardInterrupt:
	break
	except SerialException:
	sleep(1.0)
	continue
	except OSError:
	sleep(1.0)
	continue
	except:
	rospy.logwarn("Unexpected Error: %s", sys.exc_info()[0])
	client.port.close()
	sleep(1.0)
	continue

In this PR, I used rospy.on_shutdown instead of signal.signal(signal.SIGINT, self.txStopRequest), following 8fb7954 (#508) and #551
This change enables to kill serial_node.py by Ctrl-c.

Thank you very much for your help, @sktometometo

[Hommus]

Thanks for this PR @708yamaguchi!