changelog update
0.6.1
gracefully handle zeroconf node when it goes down so add_listener
doesn't throw exceptions, fixes #181.
centralising some app manager utilities in rocon_utilities.
trivial pretty print example for testing.
Merge pull request #188 from robotics-in-concert/role_manager_stub

Role manager stub updates
code for parsing global arg tags with name, value, default, refs #189
passing down args directly, references #189.
arg substitutions, closes #189
example with environment variables, #189.
Merge pull request #192 from robotics-in-concert/rocon_launch_args

Rocon launch args handling
using wallsleep clock instead of rospy.sleep. progress towards #191
Merge pull request #193 from piyushk/use_wall_clock

using wallsleep clock instead of rospy.sleep.