Push/pull pipeline example from Chapter 1 of the ZGuide

This includes the task ventilator, task worker and task sink programs from Chapter 1 (although one of the commits incorrectly mentions Chap. 2) of the 0MQ Guide in OCaml. I also included a simple makefile to help with compiling examples and fixed the wuproxy program to use the updated API. All programs were tested against the latest master.