Velocity Controllers

This PR is a little messy because it has this https://github.com/ros-controls/ros_controllers/pull/62 mixed in - once we merge that one this will look better. I'm just submitting this now so that I don't loose track of it/others know its mostly ready.

I've implemented a velocity hardware interface adapter for the ``joint_trajectory_controller`` that uses a simple PID loop for each joint to track input position and velocity and output a velocity command.

I've also implemented a velocity controller that takes as input position. Its basically a copy of the ``effort_controller/joint_position_controller``.

I've tested both of these on Baxter.