allow nil identity for ZMQ::Router
fix Message.parse to actually return the Message
for now, publish the router_endpoint as heartbeat

Ultimately the heartbeat will be a big bundle of stuff, but for now
hardcode it.
add an address to Node
rename servers config var to broadcast_servers
prefix all Registry internal vars and methods with _

avoids conflicting with config vars called id and topic
fix typo in Callbacks
update Zensu.nodes from Stethoscope

Stethoscope is only one of the potential ways to discover nodes, so it
can't own the nodes registry.
make Zensu.node a registry

Serves to configure attributes of the local node. For now, that is
mostly the signing_key (ed25519).

LocalNode is a Registry (actually a Configuration) with special behavior
for the signing_key config var. When the signing_key is set, the id is
set to the signing_key's verify key.

Using verify keys for node ids allows any message to be signed and
verified by all participants.
set identity in router from Zensu.node

If the router socket is already bound, it need to be unbound in order to
change the identity. To avoid this, always set the signing_key before
the routing_endpoint.
add examples of client and server configs