Update support for events.

After looking at this a little more, I'm confused about how this is supposed to be used. I think I was expecting a generic web api that allows a client to subscribe to events. Then this package gathers events via some generic interface which the RESTStorage objects may or may not support, and events are either delivered in real time over a websocket (or something), or batched server-side. Subscriptions are garbage collected after X minutes of client inactivity, or when the client unsubscribes.

We should provide the guarantee that events are not lost or double reported. We should probably provide, as the very first event, the state of the object in question at the time of the subscription.