Issue#56: Add `messageFree` and use it in send.

man zmq_msg_close (3) states:

    Applications should ensure that zmq_msg_close() is called
    once a message is no longer required, otherwise memory
    leaks may occur. Note that this is NOT necessary after a
    successful zmq_msg_send().

Usually when 0MQ closes the message implictly it re-initialises it as
well. Sometimes this doesn't happen though (cf. [1]) so it is possibly
the best strategy to only free the pointer on our end.

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[1] https://github.com/zeromq/libzmq/pull/721
Merge branch 'issue-56'
Update CHANGELOG and set version to 0.6.2.