Merge pull request #579 from ianbarber/master

Clean up socket_type_string
Stop ZMTP handshake when an unexpected message comes
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
Implement ZMTP/3.0 CURVE mechanism
Merge pull request #582 from hurtonm/master

Implement CurveZMQ message encryption and authentication
Merge branch 'master' of git://github.com/zeromq/libzmq