Merge pull request #231 from hintjens/master

Finishing up first round of security classes
Merge pull request #232 from hintjens/master

Finishing up with security documentation, and examples
Merge pull request #233 from hintjens/master

Added security examples final cut
Merge pull request #234 from hintjens/master

Documentation updates for 2.0.0
Merge pull request #235 from hintjens/master

Updates for ZeroMQ/4.0.0
Updated NEWS for release 2.0.1