Make the Petition Contacts group non-public

From: http://forum.civicrm.org/index.php/topic,27952.msg119349.html

Currently the Petition logic adds signers of petitions to a group whose visibility is 'Public Pages' (the default name of this group is 'Petition Contacts'). The problem with this is that, being Public, it is possible for non-signers to subscribe or unsubscribe from the list. It seems unwise to expose business internals to the public in this way.