Updated systemd service files

- Do not use a target to specify dependencies.
- Added postgresql and mysqld as optional dependencies.
  The service files work if the user has installed mysql or postgres either.
- Removed ExecStop in gitlab-unicorn.service. It is entirely wrong to do
  it in such a way, see [1]
- Added daemonize flags as it fixes systemd errors and warnings. Without
  this flags I was not able to start/stop a service properly because systemd has
  caught the wrong MainPIDs. Furthermore I had problems with service
  timouts as systemd caught the wrong return codes and considered the
  services as failed.
- Updated README.md.

[1]: http://lists.freedesktop.org/archives/systemd-devel/2014-January/016417.html