Releasing Gammu

1. Ensure that all tests pass on both Linux and Windows.
2. Update changelog and fill in release date in ChangeLog.
3. Update man pages using make update-man.
4. Run ./admin/make-release to verify release build works.
5. (optional) Test created tarballs.
6. Run ./admin/make-release branch to make final release.
7. Push created tag to GitHub.
8. Wait for AppVeyor to produce Windows binaries.
9. Import release to the website.