Command-line proxy settings

There's currently no obvious way to configure proxy settings from the command line. It's desirable, e.g. from scripts that set up SSH tunnels, to be able to modify the proxy settings. gnome-network-properties supports locations now, which allow you to store preconfigured proxy settings and choose these easily. Command-line access to gnome-network-properties locations would be sufficient, e.g.: "gnome-network-properties --location=Default" or "gnome-network-properties --location=my-ssh-tunnel".