wrapped message data into an own structure

PLEASE DON'T MERGE!!!

This pull request is used to discuss about the API changes of the Zyre:

The zyre_msg_t is only used for receiving messages, because in case of sending it actually would make things more complicated.

Maybe the API should provide an event handler for message events like ENTER, JOIN, LEAVE and EXIT and only recveive data through zyre_recv ()

//sappo