Merge pull request #576 from ahhentz/bugfix

Adds monotonic clock implementation for OSX
zbeacon.c: Remove unused variable in s_get_interface (mingw)

Related issues: #564
Merge pull request #578 from jtanx/win32

zbeacon.c: Remove unused variable in s_get_interface (mingw)
Define class export explicitly
Fixes to travis.yml
Fixed syntax error in .travis.yml
Problem: broke signal handling for some apps

Commit ce664b9 removed the ability for apps to override CZMQ's
handling of SIGINT/SIGTERM, which is a breaking change with no
workaround.

Solution: re-instate zsys_handler_set(NULL) to mean 'use an
external signal handler'. Current code is hopefully cleaner and
more obvious now, and this call can be used at any point to
disable CZMQ's internal signal handling.
Added more asserts on arguments
Cosmetic whitespace fix
Specify czmq.h explicitly
Still reworking Travis script
Further fixes to Travis script
Merge pull request #575 from hintjens/master

Define class export explicitly