add shadow methods to zctx

including shadow_pyzmq_ctx, which allows shadowing a pyzmq Context
add zsocket.from_pyzmq_socket
update to use new pyzmq underlying property

on both Context and socket

requires pyzmq 14.1.0
s/from_pyzmq_socket/shadow_pyzmq_socket/
Merge pull request #21 from minrk/pointers

Begin supporting import of pyzmq objects