add (example) systemd unit file

systemd is a modern replacement for Linux’ init system. 

systemd doesn’t require sabnzbd to fork, in fact it would make the unit file more complicated because of the manual pid-file management. Thus `--browser 0` is used to mimic the `--daemon` behaviour.

`--logging 1` is used because there’s really no downside to having debug information available via standard systemd methods (i.e. `systemctl status sabnzbd@sabuser.service` will also print the log).

Please let me know if there are any issues preventing the merge.