Merge branch 'hotfix/1.8.1'
Support newer versions of zmq (3.2 & 4.x)
Only zmq 3.1.0 (beta) was missing zmq_msg_send
Merge remote-tracking branch 'krakensden/zeromqv4' into feature/zmq-compat

Conflicts:
	src/polarssl
Merge branch 'feature/zmq-compat' into develop

Addresses PR #195
Upgrade polarssl to 1.3.0