New package: joystick_sdl

A small Christmas present to share, especially for non-Ubuntu ROS users:

For myself as a Mac user, it's long been a thorn in my side that I'm unable to plug in a joystick to drive around real and simulated robots-- when rviz, Gazebo, rqt, and everything else in ROS runs under OS X, why do I have to start a VM just to do a little teleoperation?

Please give it a try and let me know your thoughts,