zbeacon: uses only interfaces that are running

Adds a check in s_get_interface to ignore the interfaces that do not
have the IFF_UP flag set.
zbeacon: Adds virtual interface "*"

If zsys_interface() is set to “*”, make the beacon broadcast to
INADDR_BROADCAST and bind to INADDR_ANY
zbeacon: Ignore interfaces that are bonding slaves

Ignore interfaces that have the IFF_SLAVE flat set, if this flag is
defined.
Merge remote-tracking branch 'upstream/master'
Clarified zgossip protocol grammar
Minor documentation fixes
Merge pull request #573 from hintjens/master

Minor documentation changes
Merge branch 'master' of https://github.com/ahhentz/czmq
Adds monotonic clock implementation for OSX

Mac OSX does not have clock_gettime(). This adds a monotonic clock
implementation based on this stackoverflow answer

http://stackoverflow.com/questions/11680461
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