Update USB.cpp

I used the HID descriptor tool here to choose my own axis/button arrangement:
http://www.usb.org/developers/hidpage/

I also heavily based the code on this:
http://www.imaginaryindustries.com/blog/?p=80

Thank you very much Connor. To quote, "everything is under the arduino license (I think it’s GPL, but the only file I edited with a license
actually looks to be BSD-ish. Anyways, do whatever, just don’t be an ass)."