backscatterer plugin

This is a very basic pluign that checks the connecting IP against ips.backscatterer.org when the envelope-from is null or postmaster@ as per the instructions at http://www.backscatterer.org/?target=usage

This plugin is used to reject misdirected bounces and autoresponders and sender callouts from abusive systems which can happen when a local domain is spoofed and used as the envelope-from in a spam run.