Merge pull request #594 from hintjens/master

Rewrote zmonitor -> zactor for CZMQ v3
Update zinterface API

Changed the zinterface API to be only stateful, no longer a mix of
stateful and stateless, as per the discussion of #592.
Merge pull request #595 from ahhentz/zinterface

Update zinterface API
All zauth commands are now synchronous

This means caller waits for signal (zsock_wait()) after each command,
which ensures actor finishes its work before the calling thread continues.
Otherwise, the caller will run ahead and attempt to use actor functionality
that isn't ready yet. zproxy works the same way.
Rewrote zproxy class to use zactor model

Existing API is deprecated and moved to zproxy_v2. The new actor based
API is slightly more verbose than the old one. Sorry about this.