Merge pull request #580 from hurtonm/master

Stop ZMTP handshake when an unexpected message comes
Properly set mechanism to CURVE when setting a curve option
Merge pull request #581 from hintjens/master

Problem: code isn't setting mechanism to CURVE properly
Implement CurveZMQ message encryption and authentication

For protocol details, see http://rfc.zeromq.org/spec:26