Problem: need way to dump zyre state from command line

Solution: tool that connects to all zyre instances and
asks them to dump their state (zpipes_dump).

Usage:

    zpipes beacon    -- using default UDP beacons
    zpipes gossip zyre-endpoint @gossip-endpoint

Example:

    ./zpipes_dump gossip ipc://@/zyre/dump ipc://@/zgossip/hub

Fixes #96