Support for the Poller class

[flowblok]

What I’ve done:

• added the zmq_poll function
• a port of pyzmq’s _poll function
• a port of pyzmq’s Poller class
• changed the nonzero error check to check specifically for -1
  (since the nonzero assumption doesn’t hold for zmq_poll)

What I haven’t done:

• tested that the nonzero change doesn’t break anything else
• rigorously tested the Poller class to see it works properly
  (I only need it for a small script, hence I only know .register() and .poll() work fine in my scenario…)

[svpcom]

thanks