Stable eventId for NPObjectAPI.

Safari rewraps a Javascript function for each call. This means that
removeEventListener will never remove a listener set by
addEventListener. Use a sequence to set the eventId in the __fb_event_id
property on the Javascript object so that we'll be able to see it when
the object is passed in. Unfortunately, this requires removing the const
flag on getEventId.
Merge pull request #118 from byronclark/safari-event-id-cleanup

Stable Event IDs for Safari