Indigo devel diffdrive enhanced

Hi

I enhanced the ros_diff_drive to support a trip recorder and to deliver the current executed command, which can differ to the target command because of the robots acceleration limits.
This values are normally also available on real robot.
- All additional topics are only published if there is a subscriber.
- Topics names can be modified
- The new features can be tested using "roslaunch gazebo_plugins multi_robot_scenario.launch"

Greetings
Markus