Added a feedforward term for velocity commands in PID loop

In reference to @adolfo-rt 's comments last year:
https://github.com/ros-controls/ros_controllers/pull/64#discussion_r7727093
and
https://github.com/ros-controls/ros_controllers/commit/533ff43be53ea4996ffa9b2e079f889857e97483#joint_trajectory_controller-include-joint_trajectory_controller-hardware_interface_adapter-h-P14

Unfortuantly these changes were never made and when the PR was split up and refactored it was forgotten. 

Running this on Baxter, I saw huge improvements.