added xpub-xsub test for pub-sub tracing

added a test 'should support pub-sub tracing and filtering' inside test/xpub-xsub.js
based on the espresso pattern in zeromq documentation (http://zguide.zeromq.org/php:chapter5#Pub-Sub-Tracing-Espresso-Pattern)

please review test case and merge if acceptable :+1: 
otherwise, I would be happy to take feedback