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 
@hsu 
I fixed the licenses and I also added a plugin for a tricycle drive.
1.) There is also a new class GazeboRos it helps a lot on handling xml ros tags and solving the correct model name, have a look on the constructor of the tricycle drive how easy it is now to read parameters. 
2.) The Tricycle drive works but it will be enhanced soon with a pid controller. There is also a launch file to test the drive.

Greetings
Markus