installing joint state publisher so that it can be used by rosrun

As of right now, `setup.py` is installing `joint_state_publisher` into the global bin folder instead of joint_state_publisher's bin folder. This prevents `rosrun` from running joint_state_publisher. With this patch, it will be installed in both places. 

See https://github.com/ros/xacro/issues/1 for the same discussion on xacro. An alternative is to change setup.py to install correctly. I have followed xacro's current logic in this patch