Started implementation of gossip discovery

Selftest is in zyre_node_test

Body of test is still empty, we do gossip setup but don't yet handle
DELIVER messages from gossip network. This commit is meant to show
the Zyre API for gossip discovery.
Connect properly to peers via gossip protocol

Now handles discovery via gossip network as an alternative to beaconing.
The zyre_test method shows an example of this.

This means we can now run Zyre clusters over inproc or IPC rather than
only TCP.
Merge pull request #220 from hintjens/master

Gossip-based discovery now works