[imp #2] First implementation of Broadcast messages

Not quite satifacting: it is possible to create and send
synchronization and broadcast message interlaced in one command
(see scenario "broadcast messages and synchronization commands interlaced").
As a consequence, such messages are difficult to produce and process since:
- We need to foresee the server revision for the broadcast message if sent after
  a synchronisation message.
- Server response is not natural, since it is composed of two same messages
  containing the stack (one for the actual synchronization message and one for
  the broadcast message).
As a side note, the original mobwrite protocol is more simple concerning
broadcast message: a client can only send one broadcast message (which can be
overwritten or blanked by future broadcast message), and it is only associated
with a file, not with a (file,revision) tuple.
[imp #2] Better implementation of messages.

In this implementation, a broadcast message has been 'elevated' to the same
level as a SyncCommand. Thus, it allows a more natural and easier processing of
such messages.