zpubsub class

zpubsub is a relatively general purpose framework for pubsub (over TCP), handling subscriptions asynchronously using a zactor. It uses zbeacon to advertise publishers, and subscribers automatically connect to all discovered publishers. The doc and examples should be relatively easy to follow. (Disclaimer: This is my first attempt at contributing to czmq. Any comments and constructive criticisms are welcome! I have read and hopefully understood the C4 process documents.)