Implement server-side message-passing

[amcgee]

This is required for realtime client-side data updates (issue #15) since most new information (i.e. new reports) comes in through the gateway.js process, not the api.js process. We could use PGSQL's Trigger functionality to capture any new inserts or updates on specific tables, or implement a separate message-passing scheme using something like ZeroMQ.