Merge pull request #146 from hintjens/master

Part of an ongoing cleanup of server/client codecs
Merge pull request #147 from hintjens/master

Massive breaking changes for C server/codec
Problem: zproto_client_c not using v2 codec

Solution: switch to new codec style in same way as for server.

This also makes the code more consistent; it is always self->message
rather than various other names.