ArchiveRunner has Pipermail artifacts

When the original message has a Date header, ArchiveRunner._dispose() will check to see if the date should be trusted or clobbered. This code still references the out of date [archiver.pipermail]clobber_date_policy value. The config var needs to be moved or renamed and the functionality re-implemented (and tested!). For now, I'm going to disable this bit of code but it should be re-addressed for beta2.