merge in of systemd-crontab-generator

Hi,

This is a proposal to merge systemd-crontab-generator (from @kstep ) into systemd-cron
to make it POSIX-compliant and close bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=752376 .

The changes to debian/ folder follows in an separate pull request.
systemd-crontab-generator is licensed "CC-BY", this is referenced in debian/copyright