Problem: does not handle dropped writer properly

When waiting for read, blocks if the writer drops. Solution is to
return a short read, or read_end if there's zero data.

Fixes #41.
Rebuilt latest sources
CZMQ is unstable, so use stable release 2.2.0
Using ZeroMQ stable 4.x
Merge pull request #42 from hintjens/master

Problem: does not handle dropped writer properly