Begin supporting import of pyzmq objects

- add shadow methods to zctx
- add zsocket.shadow_pyzmq_socket (depends on zeromq/pyzmq#488)

I'm working on the other direction in zeromq/pyzmq#488, but I wanted to open this now so I can get some feedback.

Some questions:

- where should this functionality live, and what should it be called?
- should pyczmq->pyzmq live in pyzmq or pyczmq, and vice versa, or all in pyczmq, or all in pyzmq?
- should zctx.shadow_zmq_ctx support a pyzmq Context object, or just be a strict wrapper?