HID+CDC

I've changed the usb functionality to be a composite HID+CDC device. As far as I can tell, there is no change of functionality for now. Windows will need a modified .inf file I think to see the two devices, or at least a change of PID. Linux works just fine, and I've tested that I can do usb-serial and HID.

The usb code is not yet doxygen formatted. This is because I just merged the existing serial code with another teensy example, so the code is no less commented than before. I hope that is ok for now.

I defined F_CPU to stop a compiler warning, which should have been a separate commit, but I am a bit rusty on git, and can't remember how to fix that.