usb_device support for ATmega microcontrollers.

NOTE: This is an experimental library, and certainly not production-ready. Expect bugs, and please help out by reporting them if you find any! If you need absolute reliability, consider a more mature framework like LUFA.

Currently supports:

• ATmega16u4/32u4

Untested but probably close to being supported:

• ATmega8u2/16u2/32u2 - USB peripheral is very similar to m16u4.

The arduino_keyboard example is a "Hello World" example that can be run on an Arduino Leonardo. Usage is documented in the example file.