Added link to zmq_msg_gets
Problem: zmq_socket_monitor code is dirty

Specifically:

* zmq_event_t should not be used internally in libzmq, it was
  meant to be an outward facing structure.

* In 4.x, zmq_event_t does not correspond to monitor events, so
  I removed the structure entirely.

* man page for zmq_socket_monitor is incomplete and the example
  code was particularly nasty.

* test_monitor.cpp needed rewriting, it was not clean.
Merge pull request #985 from hintjens/master

Cleaned up socket monitor code, tests, and man page