Creates a new message of the specified type, in association to a parent (incoming message) and optional metadata and content.
function newMessage(type, parentMessage, content, metadata) {
  var header = {
    msg_type: type,
    msg_id: uuid.v4(),
    session: parentMessage.header.session,
    username: parentMessage.header.username
  };

  metadata = metadata || {};
  content = content || {};
  return createMessage(parentMessage.identities, header, parentMessage.header,
                       metadata, content);
}