Add Router module

This module provides some convenience functions for communicating over a
router socket.
EAGAIN and EINTR map to Unix.Unix_error

Update init code in tests to use `Context` functions.
Merge pull request #5 from andrewray/zmq4

Update to latest version of ZMQ4
Expose argument ?more of ZMQ.Socket.send.
Merge pull request #7 from whitequark/master

Expose argument ?more of ZMQ.Socket.send.
Major version bump
Merge branch 'master' into release
Build system update for v2.0.0