Debian packaging

This pull request adds debian packaging. As per #49 libexec/* has been moved to lib/github-backup-utils/*. While building in a chroot I noticed that the tests assume a configuration of git which specifies user.e-mail, this is assumption is removed in c252898.

Feel free to modify the maintainer fields, debian changelog release, and any other such things.

Running `debuild` or `sbuild`(1) will run the tests and build a debian package.

1: this does require some system configuration.

Closes #49.