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
Looks sketchy. Please remove this and use `angles::normalize_angle` instead: http://docs.ros.org/api/angles/html/

(And watch out about inserting functions between another function and its comment block...)