Add ability to disable event monitor

This change provides an ability to cleanly shutdown the socket event monitor socket. In libzmq src/socket_base.cpp#L1207 it can be observed that supplying an addr of NULL is interpreted as a signal to shutdown the event monitor socket. This change is in line with what the CZMQ event monitor does on shutdown.

While I have made the updates for both Cython and CFFI, I have only tested the Cython code. I have updated the monitor example as well as the test_monintor.

My sublime-text setup automatically trims trailing whitespace which has made the change appear larger that it really is.