Added Message-type IUs (using 'MESSAGE' access).

Important: Java version does not yet contain
  Messages (or Retraction, for that matter)
Note: Messages are received and routed to the
  handlers with the special new MESSAGE event
  type, as opposed to ADDED. This decision is
  of course open for discussion.
Additional bugfix: Typecasting conflict in C++
  payloads fixed. Reverted to cstdlib casts
  to avoid bulky exceptions being thrown.