Minimal USB stack for XMEGA micros

Originally a fork of https://github.com/kevinmehall/usb, but it has since diverged significantly since then.

• WCID support. Note that you need at least one endpoint for WCID to work.
• HID support.
• Bulk endpoint support, can achive about 8Mb/sec.
• DFU runtime support.

See notes.txt for more details.

GPL v3 licence.