Moderation rules priority

As discussed here [1] on the mailing-list, an incoming email can have a member and a non-member amongst its senders.
[1] http://www.mail-archive.com/mailman-developers%40python.org/msg14328.html

In the current setup, the nonmember "wins" and the message is held.
Here is a patch implementing my suggestion, which got Barry and Mark's approvals. It does not implement Stephen's suggestion which would require a small refactoring but may be cleaner (follows the DRY principle better).
The patch includes a unit test.