add velocity interface for joint_trajectory_controller

...in separate feature branch

This feature has been provided by @davetcoleman
Please remove the 'Notice the \p gains entry' phrase.

If you want to add an example usage, could you please mimic the example used for the effort adapter as closely as possible?, so it becomes straightforward to spot the differences. With this I mean preserving the joint count and names, as well as constraint specification. The only things that should change are the controller type (namespace prefix) and the PID gains, which are not needed in for position joints.