Added actionlib interface to forward_joint_group_command_controller

[egordon]

In service of https://github.com/ros-controls/ros_controllers/issues/568

Prerequisite: https://github.com/ros-controls/control_msgs/pull/60

Tested via velocity_controllers/JointGroupVelocityController on Kinova JACO2

[egordon]

Test code (Python 3): test_JointGroupCommandAction.txt

Tested behaviors:

• actionlib velocity commands (0.1, -0.1) with timeouts (0, 1, 5, 15)
• Preempted actionlib goal using the /command subscriber interface during goal execution