Added support for systemd and Arch Linux

I did a systemd service unit that simply wraps to the actual init script (note: the systemd service unit runs /usr/local/bin/msm not /etc/init.d/msm).

I also made an installation script for archlinux. Everything works except for the download of the systemd service file, because it links to your master branch, where the file doesn't exist. It will work however as soon as the file is in your master branch (if you decide to do so).