[IndexedDB] IDBEvent and Event

Hey all,

I think that IDBEvent needs to inherit from Event [1] in order for us to 
properly inherit from EventTarget in IDBRequest.  Specifically, 
EventTarget takes an EventListener [2] which has a method, handleEvent, 
that takes an Event object.  I'm not sure this makes sense for us 
though, so I figured I'd start a discussion before filing the bug.

Cheers,

Shawn

[1] http://www.w3.org/TR/DOM-Level-3-Events/#interface-Event
[2] http://www.w3.org/TR/DOM-Level-3-Events/#interface-EventListener