added publisher for wheel joints.

In the dead_rockening, wheel joint states are computed from encoder data and published on `joint_states` topic. 
fixes https://github.com/g/grizzly/issues/2
I should have alerted you to this earlier, but you can use [vectorFromDriveMessage](http://docs.ros.org/hydro/api/grizzly_msgs/html/namespacegrizzly__msgs.html#aeb1f2b9e670fb638bc9e2accb4738873) here to eliminate this logic.