Zend_Queue will also accept your own message
iterator class. The message iterator class is used to return
messages from
Zend_Queue_Adapter_Abstract::recieve().
Zend_Queue_Abstract::receive() should always
return a container class like
Zend_Queue_Message_Iterator, even if there is
only one message.
See the example filename in
tests/Zend/Queue/Custom/Messages.php.