create systemd service file

[boyska]

For debian package (see #229 ) but also more generally, it's nice if startup scripts are already provided. While this will always be a bit distro-specific, I think it's better if some basic file is put in this upstream repository.

[boyska]

See http://git.lattuga.net/boyska/deb-libreant/src/master/debian/python-libreant.service

But it misses some hardening still:

• is PrivateTmp ok? probably
• can ProtectHome and ProtectSystem be used? probably
• can PrivateDevices be used? probably
• can PrivateNetwork be used? not when agherant is to be used!
• can we chroot to /var/lib/libreant/? who knows!