environment-aware `npm start`

This is a pattern I learned at Heroku for making `npm start` do the right thing depending on your environment.

I know this change won't interfere with deployments today because `ndm` doesn't currently use `npm start`, but I want @ceejbot and @bcoe to see this anyway. What do you guys think? How would I set `NODE_ENV` to "production" in `service.json`?

cc @rockbot, who might want to do this in newww too.