Initial commit for adding xpub-xsub test

first write of test case for pub-sub tracing
known as the espresso pattern
(http://zguide.zeromq.org/php:chapter5#Pub-Sub-Tracing-Espresso-Pattern)
Refactoring xpub-xsub test

cleanup and refactoring of test case
Clean up code in xpub-xsub test

quick edits to comments and unsubscribe timeout
Added version check to xpub-xsub test

zmq version has to be greater or equal to 3.1.0
(xpub, xsub not supported in zmq 2.x.x)

also commit for travis
Moved xpub xsub socket creation

Creating xpub and xsub sockets after version check, not in beforeEach
Refactoring xpub-xsub test

removed beforeEach function in efforts to resolve current travis build
issues (which are not reproducible anyways)
refactored away from XSubXPubProxy function
use different inproc name to avoid INUSE error when called async in testing
...
Fix indentation issues

changing from a mix of tabs and spaces to all spaces
changing from 4-space wide indents to 2-spaces.
Merge pull request #352 from bluebery/xpub-xsub-test

added xpub-xsub test for pub-sub tracing