Merge pull request #123 from toshic/stable

Improvements in C++ and python bindings
Depend on libzmq-dev
Merge branch 'stable' of github.com:reverbrain/elliptics into stable
v2.20.6.7
Depend on zmq < 3.0
In C++ bulk_write method return real error code instead of -EIO
Changed bulk_write error report logic

Only return wait error (usually -ETIMEOUT/-110) if wait internal
status (assigned in write completion handler) was set to something
except -ENXIO.

-ENXIO is set by default and means no transactions were sent