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!
Because I need the number of joints in compilation time here:
https://github.com/efernandez/ros_controllers/commit/b287fdd1ae9775701edc6d3793fb3aa17a948c99#diff-3e902f25a0769147d5d4219e210e792eR130