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!
I would try to keep the number of files to a minimum and put this single line in the 2 xacros. In the end, this is a one-liner as well as the include for it, but there's more bloat with a new file.