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
Whoops, this isn't going to compile, I don't think.

Also, are these variables necessary? What about just a single instance of JointState which you populate? It contains vectors for the variable-length fields; you just resize them.