Hackathon project: Add an event server that writes/reads to/from a unix socket on a path

[nathansobo]

Started on this at the Boulder Startup Week Hackathon with @mweitzel. The idea is a dead simple extensibility scheme for basic events.

Set event_server: "/path/to/domain.socket" in the settings. We can access the EventServer global across the codebase to publish (an maybe later subscribe to) events.

Release Notes:

• N/A