Add support for graceful shutdowns

[totherik]

Not totally sure where the behavior belongs, but we want to be able to stop the server from accepting connections and allowing it to drain existing connections before exiting. We can do this a number of ways, but adding a hook for a process signal seems most straightforward:

Add hook for an agreed upon signal (SIGTERM?)
Stop server (prevent new connections)
Add 'Connection: close' to all existing connection responses after server.close().
Once server closes, exit process with appropriate status code

[lensam69]

Here's some interesting points to consider :
http://blog.argteam.com/coding/hardening-node-js-for-production-part-3-zero-downtime-deployments-with-nginx/

[tlivings]

Should listen for both SIGINT (Control-C) as well as SIGTERM.

[jeffharrell]

This is now resolved as part of #21.