Overhaul USB HID for Leonardo and Micro

This pull request includes the features from pull request 1488 and pull request 1391. I've somewhat overhauled the code to better match the surrounding code and a few features.

I've also added code examples for the less discoverable features.

If this pull request isn't "right" yet, I'd love feedback so I can improve it.

> > + size_t Keyboard_::pressKeycode(uint8_t k)
> > + {
> 
> Why Size_t? The report can have up to 6 keys, so uint8_t would be enough. I am not sure but i remember size_t is a 2 byte value on AVR?

IIRC, it's because it helped support usb NKRO. (Which became a 2 byte change with the generalizations)