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.
These virtual functions eat a LOT of flash, even if they are not used. I'd rather make them normal again or even inline the whole class in the header. Maybe this is the reason for the big delta too.