Merge pull request #553 from hintjens/master

Some changes to building and packaging
Abstract security mechanism
Implement ZMTP/3.0 PLAIN mechanism

This implements protocol handshake.
We still need to design and implement 1) API changes so a user
can set username and password, and 2) a mechanism for engine
to authenticate users.
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