Sorry for the huge checkin message - I was only intending to implement #496356

Sorry for the huge checkin message - I was only intending to implement #496356
but I found a number of places where things had been broken by transactions:
 . modified ROUNDUPDBSENDMAILDEBUG to be SENDMAILDEBUG and hold a filename
   for _all_ roundup-generated smtp messages to be sent to.
 . the transaction cache had broken the roundupdb.Class set() reactors
 . newly-created author users in the mailgw weren't being committed to the db

Stuff that made it into CHANGES.txt (ie. the stuff I was actually working
on when I found that stuff :):
 . #496356 ] Use threading in messages
 . detectors were being registered multiple times
 . added tests for mailgw
 . much better attaching of erroneous messages in the mail gateway

git-svn-id: http://svn.roundup-tracker.org/svnroot/roundup/trunk@487 57a73879-2fb5-44c3-a270-3262357dd7e2