Merge pull request #350 from sjohnr/master

Revert "Raise exception if zmq_poll returns error."
Changed ZMsg to return null when no data available (e.g. closed
context), with unit test
Changed ctx.destroy() to ctx.close()
Added error message to assertion
Added constructor to ZMQException and cleaned up change to ZFrame
Merge pull request #352 from sjohnr/issue/349

Change behavior of ZMsg when context is closed
Revert "Merge pull request #350 from sjohnr/master"

This reverts commit ab8517e7d5b780980f516f82b01350052f9266e9, reversing
changes made to 6c8d4160c7901abaaea8cbcf29857994d305b7f5.
Ensure sockets are connected before we attempt to disconncet them.

Otherwise this test fails on OSX 10.9.4, zmq 4.0.3
Ensure that exception is thrown when polling invalid socket.
Merge pull request #353 from GreatFruitOmsk/master

Ensure that exception is thrown when polling invalid sockets.