Merge pull request #552 from hurtonm/master

Implement ZMTP/3.0 NULL mechanism
Added configure check for libsodium
Fixed packaging for Windows - was missing errno sources
Use correct libsodium call for detection
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.