Added support for gossip discovery

Fixes #86

Configuration for broker (see zbroker.cfg as example) now has this
structure:

zyre
    name = xyz          #   Default = random name
    discovery = gossip  #   beacon | gossip
    gossip
        endpoint = ipc://@/zyre/two
        connect = ipc://@/zgossip/hub

The discovery model is based on CZMQ/zgossip. That is an underlay network
which exchanges config information across a loosely-connected network.
Each node must either bind (zyre/gossip/bind) or/and connect to other
nodes (zyre/gossip/connect). I'm testing with ipc:// but in reality you'd
use tcp://.

Separately, the node has to specify its endpoints for clustering:

    zyre/gossip/endpoint -> node binds to this
    zyre/gossip/announce -> node announces this to other nodes

Typically you'd use tcp://eth0:5670 as the endpoint (port 5670 is the IANA
port for Zyre), and tcp://thishostname:5670 as the announced endpoint for
other peers.