Gazebo ROS joint state publisher added

Hi

I wrote a gazebo joint state publisher to enable gazebo to publish non fixed joint values.
See http://answers.gazebosim.org/question/4863/ros-differential-drive-controller-does-not-publish/?answer=4875#post-id-4875
Jose Luis Rivero asked me to send a pull request :-)

Greetings
Markus


@testing
I tested the plug-in only for continuous joint types.
@ToDo
Currently the plug-in is able to publish multiple joint values defined in SDF or URDF but the list of joint values are separated by commas at shown in the added header file and this can be made better with XML

Greetings 
BSD is ok with me