Add systemd unit file

This systemd unit file is basically the one can be found in Gentoo, only the description was modified to match the 'official' description. I also modified the install.sh script to install the systemd unit file into /usr/lib/systemd/system directory.

But as I'm not using the install script itself, I was not sure what other changes you might need in it (e.g. MY_VERSION bump, pre-install checks or post-install notes, etc.). So please review the proposed changes and let me know if you need further modifications in order to merge.