y2makepot - added support for ruby gettext-3.0.0

- verify the POT file syntax by running `msgfmt --check` to ensure
  that the output is GNU gettext compatible
- added `--copyright-holder` option
- 3.0.5
Merge pull request #25 from yast/rxgettext_update

y2makepot - added support for ruby gettext-3.0.0