Basic clustering protocol implemented

- servers detect each other using Zyre
- readers and writers synchronize using a SHOUT/WHISPER handshake
- added zpipes_test_cluster test case
- still at Hello, World level

Next steps are to expand test case and ensure all semantics work.
There are still grey areas, e.g. if multiple clients try to work
with same pipes at same time.

Clustering protocol needs to be properly defined and generated,
and more work done on resolving multiple-writer/multple-reader
scenarios. I think solving N will solve 1 better.
Merge pull request #34 from hintjens/master

Basic clustering protocol implemented