Add license + copyright to all files

[ildyria]

This will give us Gold badge :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.06%. Comparing base (4258b84) to head (4357ca1).
Report is 18 commits behind head on master.

Additional details and impacted files

[d7415]

I wonder whether the end date on the copyright is needed. That's a big patch every year. But other than that sure.

[ildyria]

Not specifically needed but it helps possible to see that the project is alive too. 🙂