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.
Why do you think that someone is using "nothing of the existing API" 
only because one or two methods get overwritten? That's just not the 
case.

> Why not create a whole new class if you use nothing of the existing
> API then? You simply overwrite the whole class implementation.