Add systemd service file to contrib

Supersedes #887 to add a systemd unit file which will run as a user and
whose gollum path and options can be changed as needed.
Prepare contrib directory

Move openrc files to contrib/ in conjunction with systemd service.
Update gemspec to bundle contrib/ into the gem.
Merge pull request #893 from tuftedocelot/add-systemd

Add systemd service file to contrib and move openrc files to contrib directory as well.