Release 3.1.0 (GothenburgBitFactory#3574)

Release 3.0.2 (GothenburgBitFactory#3394)

Update for 3.0.1 (GothenburgBitFactory#3382)

update for 3.0.1

Prepare for 3.0.0 release (GothenburgBitFactory#3287)

README: Update version reference

cmake: Bump project version to 2.6.1

README: Mark GnuTLS as optional

cmake: Prevent nested tarballs (or other archives) in the release

This happened dring the release of the version 2.5.2.

holidays: Update holidays using newest holidata.net data

Remove outdated holiday files that could not be updated.

Task Release 2.5.1