Merge pull request #570 from hintjens/master

Packaging of probe function
Rename terminated->pipe_terminated
Minor cleanups
Implement ZAP and integrate it with PLAIN mechanism
Merge pull request #571 from hurtonm/master

Add ZAP support
Small fixes to documentation

- REQ and REP sockets don't have HWM issues
- ZMQ_DONTWAIT applies to DEALER and PUSH only
Updated ZAP request for changed protocol draft

- username and password sent as two string frames
- fixed test case to match