Add systemd unit and service files

[clawoflight]

Since #139 has been solved, it's now possible to do away with the GUI entirely after initial configuration.

I have augmented the Arch Linux package with a systemd unit file and corresponding timer in order to run a daily background sync.

All that needs to be done is install them to /usr/lib/systemd/user, and users can systemd --user enable gpgsync.timer to take care of periodic synchronization.

The files are here, feel free to take them and adapt as necessary.