Restore/improve CI

[Athari]

All parts of Travis-based CI died. Fix everything.

• Run phpUnit on all supported PHP versions
  • b221abe
• Run tests during CI
  • Add GitHub Actions CI workflow #69
• Run tests during CI on all supported PHP versions
  • Add PHP 7.0 and 7.1 to CI workflow matrix #71
  • Update CI workflow to include PHP 8.0-8.4 #72
  • 4c69dae
• Upload coverage to Coveralls
  • 9a0c87c
• Run phpDocumentor
  • Documentation / ReadMe: MOAR #52