Adds support for multiple wheel joints per side

This might help to give support for skid-steer robots, as requested in https://github.com/ros-controls/ros_controllers/issues/100

Note that the code compiles, but no test has been done yet!
These checks are getting quite long. Can we couple them with the previous one? In the end it's the same error, wrong parameters for wheel names. Or a one-liner ROS_ASSERT could also work here.