Use server-sent events

This is the implementation of #14 for harpoon-agent. It's also the first unit test :)

For now, the previous event interfaces still exist in the agent lib, and are still implemented by the agent's internal types. @peterbourgon, should those changes take place here? Or as a part of the scheduler changes?
Ideally? Yes. But this channel comes from [`net/http`](http://golang.org/pkg/net/http/#CloseNotifier). The `eventsource` package just passes this along, and is probably not worth wrapping.