Add event set management

Thoughts on this? Basically it's an implementation of the event create/delete endpoints in 1.1 on both custom sets and instances.

* On the custom set add/remove methods, there's an optional 'time' param that inserts an event at that time. Is the new code in those methods styled how you'd like?

* Instance events can be created either by hand with `Fauna::Event.new(...)` or with the `instance.new_event(...)` method.

* Persisted events can be deleted, and new events can be saved.

* `Fauna::Event` gets an `event_ref` method that returns the full ref for that event in the set it belongs to, without interfering with its existing `ref` method. I went back and forth on naming this method. `event_set_ref` might be better, not sure.

* Still needs tests.