Add a pub/sub notification mechanism and make all classes listen

[taoeffect]

So right now things aren't as great as they could be in DNSChain-code land, but we've got radical improvements lined up (#80, #75).

Because we're not using a events/notifcations (the pub/sub model), we've got code like this:

shutdown: -> [@nmc, @dns, @http].forEach (s) -> s?.shutdown?()

But that's kinda lame because it "central management" and requires DNSChain to be updated for every single new server that's added. Instead, as per #80, we can implement in blockchain.coffee (the template for all supported blockchains) a notification listener for ShutdownNotification, and have the individual servers clean up for themselves.

Then DNSChain's shutdown: method would simply post that notification, wait a bit (asynchronously) and then process.exit.

NodeJS already has an event system that we can use to implement this.

Edit: Note, this part from app.coffee might need a small update.

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