[installation script] GitLab 5.4 - Ubuntu Server 12.04 - Apache (support relative url)

This script is not perfect but does a standard installation of GitLab 5.4 with Apache2. After the successful execution of the script GitLab will be available under http://servername/. I tried to configure Apache2 so that GitLab runs under http://servername/gitlab but I was not able to get it to work. I added a couple TODO lines where I think the script could be improved.