add get_credentials method, reinstate support for old message handler override, clean up
add get_credentials method, reinstate support for old message handler override, clean up

tcmitchell/geni-ch,MarshallBrinn/geni-ch,tcmitchell/geni-ch,ahelsing/geni-ch,MarshallBrinn/geni-ch,ahelsing/geni-ch