Add support for heartbeats

[yoshuawuyts]

Process managers should be able to ping internal services to figure out how they're doing, and if they're frozen or spinning be able to reboot 'em. We should figure out a good way of doing this.

Some reference resources:

• https://github.com/watson/opsgenie-node

cc/ @davidmarkclements do you know of any prior art / any patterns you know of?

[davidmarkclements]

It may be overkill but the SWIM protocol is primarily for heartbeats - there's http://npm.im/swim also check out @mcollina upring http://npm.im/upring as a reference

[yoshuawuyts]

Thinking that republishing to a DHT might actually be a good one (: