Add ElggEvent as a core PHP object

[ewinslow]

Events are a pretty standard social concept, and I would argue that core support for them makes a lot of sense. In addition, true core support for events would effectively provide a standard for the implementation, which means event plugins would be more interchangeable/compatible. Up until now, all the events plugins I've seen are completely incompatible

I might even go so far as to say they should be a top level entity since they have standard fields that could really use their own table: name, description, dtstart, dtend, duration, location, event type, but I'm certainly fine with just making them a subtype of object first.