Add queue_size to diagnostic_updater for Python.

cf. http://wiki.ros.org/rospy/Overview/Publishers%20and%20Subscribers#queue_size:_publish.28.29_behavior_and_queuing
Merge pull request #35 from mikepurvis/patch-2

Add queue_size to diagnostic_updater for Python.
Merge branch 'hydro-devel' into indigo-devel