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
Split out 'code' target as independent.