Create a monotonic clock function, use where time offsets are computed


Problem:  If the system time is set backwards while a czmq application is
          running that has timers set on a zloop, those timers will stop
          until the system time reaches its previous value.

Solution: Introduce a monotonic clock that is not affected by system time
          changes.  Use this clock where time offsets are computed.


I've added a simple test to the self test.  I built and ran on both Ubuntu 14.04 and Windows 7.