Add pyzmq support

[veegee]

We need to add pyzmq support. Good starting points are as follows:

• The eventlet zmq support module, but this might be a bit outdated.
• The pyzmq project itself, which has integrated support for gevent (import zmq.green as zmq), see https://zeromq.github.io/pyzmq/eventloop.html

We can do a pretty quick port from the gevent code, since the structure is almost the same. The only difference is that guv uses trampoline() instead of the lower level, direct constructs that gevent is using; if anything, this makes it easier.