Check whether all PHP files in `src/` have the required license header
Merge pull request #16 from SURFnet/maintenance/license-headers

During pre-commit, check whether all PHP files in `src/` have the required license header
Correct InvalidArgumentException::invalidType() $parameterName type.
Use more verbose ID names in events (identityId instead of id).
Allow verification of Yubikey token, unless user already has a token.
Remove unused use statement.
Make event fields public.
Make aggregate root applyFooEvent methods protected.
Create AbstractCommand that sports the mandatory $UUID field.
Handle VerifyYubikeySecondFactorCommand.
Project second factors.
Fix event_stream table primary key.
Add missing license header.
Add Event suffix to YubikeySecondFactorVerified event.
Fix code style.
Test Identity aggregate root.