Xbox 360 Controller OS X and trigger support

I modified the xbox360 driver to add trigger support.  Also, I wasn't sure why the button map and axis numbers were set to the wrong values with the original code compared to the values I saw in testing.  I'm assuming this was due to a difference in USB driver support, so I created a separate map for my USB driver and used the additional_params from the Artoo::Driver class to access it.  I kept the original button and axis numbers in the default map.

I added information about the third-party USB driver to the readme as well as details about the trigger support.