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!
With ROS_ASSERT the controller_manager crashes (in simulation this also means that gazebo crashes); I had to compile with CMAKE_BUILD_TYPE=Debug, because in Release the ASSERT macros are removed.