mam: various improvments: - put common namespaces ton const - VAL_RSM_MAX_DEFAULT can be None if default limit is not wanted - ItemDate now has a 'date' attribute - MAMService is MonkeyPatched the same way as PubSubService to handle PEP - fixed error mapping in mam module - PEP is handled - properly manage date in a payload independent way - when PEP is used, send privileged messages