Problem: need a basic plug-in model for streams

Solution: use an actor API with async message flow. Implemented by the
mlm_stream_simple actor. See STREAM.md for protocol details.
Problem: need continuous integration tests

Solution: make Travis CI integration
Problem: Travis script does not build Zyre so configure fails

Solution: we will use Zyre later; rather than remove this from the configure
script I've added it into .travis.yml.
Added missing include of labs/zbits.h
Fixed use of include files
Problem client 'attach' and 'subscribe' are clumsy names

Solution: use 'produce' and 'consume', these work better.
Merge pull request #12 from hintjens/master

Problem: need a basic plug-in model for streams