Merge pull request #554 from hurtonm/master

Implement ZMTP/3.0 PLAIN mechanism
Fixed sizeof usage in man pages
Added configuration for PLAIN security

* ZMQ_PLAIN_SERVER, ZMQ_PLAIN_USERNAME, ZMQ_PLAIN_PASSWORD options
* Man page changes to zmq_setsockopt and zmq_getsockopt
* Man pages for ZMQ_NULL, ZMQ_PLAIN, and ZMQ_CURVE
* Test program test_security
Merge pull request #555 from hintjens/master

Added options for PLAIN security
Use socket options to select security mechanism