Problem: can't play Matryoshka with czmq

Solution: add helper functions to add and pop zmsg_t from other zmsg_t.
Those are encoded and added as extra frames.
Updated for release 3.0.0
Problem: whitespace errors

Solution: fixed all I've spotted.
Merge pull request #742 from vyskocilm/master

Problem: can't play Matryoshka with czmq
Added zbase64 class.
Split out 'code' target as independent.
Merge pull request #745 from pmienk/master

Issue #743 fix
Merge pull request #744 from lovmoen/master

Added zbase64 class.
Problem: zsock_recv 'm' assumes initialized message

This is inconsistent with all other picture formats and leads to
surprising code.

Solution: always initialize message before reading off socket.

If we need an "append to message" picture that should be a special
case, e.g. using 'M' as picture.