Read timezone from /etc/timezone.

According to [this](https://github.com/fmbiete/Z-Push-contrib/wiki/Example-Configuration#main-basic-configuration), the timezone for z-push might be important. Reading `/etc/timezone` and using it in z-push' config.php seems to be a good idea.

I had to change the delimiter for sed to compensate the `/` in the timezones name.