SDL joystick input handling implementation

I did an implementation for XBox 360-like gamepads. Seems to work OK and I have tested it with both an XBox360 wireless gamepad and a Logitech F710 wireless gamepad. Implementation is SDL2 safe. 

The main limitation is that SDL button/axis number are hard coded to Doom3 names. If you use a non-Xbox360 gamepad this may have unwanted results. Also, to bind button/axis to actions the files joy_lefty.cfg and joy_xbox360_0.cfg must be manually edited. I'm willing to implement a way of solving both problems if you suggest a proper way of doing it (for example, a configuration file to map SDL button/axis numbers to Doom3 names and a Doom3 menu entry for joystick remapping/binding).

This should close issue #64.

Please try it and enjoy.