Apply REUSE recommendations for code, documentation, and examples licensing.

[Flameeyes]

The REUSE specifications are meant to make
it explicit and easier to apply code licensing information for a
project. The provided lint tool makes it easy to ensure all the
content (code and not code) is tagged.

I'm sending this pull request as a request for comment on whether this
approach would be accepted for all or at least most of the Adafruit
repositories.

My reason to look into doing this, beside a personal
preference,
is that I was looking at publishing a quick script based off the
example from the documentation, and I had to dig a bit to find the
correct license and copyright information for it.

So I thought I would suggest this as an option — and volunteer to help
implementing this, of course. It can also be set up with
pre-commit as well, which should make it
harder for regressions to appear.

Important notes:

• All the code (and documentation) that otherwise didn't have an explicit
  license headers have been tagged with @ladyada's copyright as per the
  LICENSE file.
• The favicon.ico file has been tagged with MIT license and the same
  copyright information. I do believe this is incorrect, and would like to
  know what the correct information should be.
• All configuration files have been tagged with ladyada's copyright and
  Unlicense. The current REUSE recommendation is to use
  CC0-1.0, but that has… side effects. There's some discussion in
  Alter CC0-1.0 recommendation for uncopyrightable files fsfe/reuse-docs#62 about the recommendation for
  likely-uncopyrightable files (such as configuration files).

[tannewt]

This looks good to me! I know the copyrights aren't perfect but authors who changed things can create PRs to add their names if they like.