Don't require czmq to be installed for labs
Fix and run labs test suite
Enable labs for travis
Check for pkg-config before using it
Allow NULL contexts in zbeacon_new again.

zbeacon_new had the option to pass NULL as context (it is even in the
docs). This option was lost in the transition from zbeacon to
zbeacon_v2. This commit restores this option.
Merge pull request #725 from Q-Leap-Networks/pull-labs-autotest

Fix test suite for labs and enable labs in travis builds
Problem: Labs tests shouldn't be enabled in travis

The labs contain code that is unstable hence causing a lot of errors.
But that's ok as the labs area has been create to mature unstable code.
Error in the labs shouldn't bother the main czmq project.

Solution: Remove labs testing from travis build.
Merge pull request #727 from sappo/master

Problem: Labs tests shouldn't be enabled in travis
Merge pull request #726 from ahhentz/bugfix

Allow NULL contexts in zbeacon_new again.