Just a FYI if anyone else will stuck trying to enable this. There's no debug print out for the serial connection in pymysensors as it appears. You need to add the debugs in the code on your own.
Here's the diff:
--- mysensors/mysensors.py.orig 2016-11-19 17:51:39.000000000 +0100
+++ mysensors/mysensors.py 2016-11-19 18:27:43.705631204 +0100
@@ -483,6 +483,7 @@
'Error decoding message from gateway, '
'probably received bad byte.')
+ _LOGGER.debug('Received %s', string.rstrip())
self.disconnect() # Disconnect after stop event is set
@@ -492,6 +493,7 @@
# Lock to make sure only one thread writes at a time to serial port.
+ _LOGGER.debug('Sending %s', message.rstrip())