add the _Event<T> helper class to make proxy event streams lazy

Disposable : `dispose` now returns a future as it may be async (closing
streams).
_Event objects are lazily created, and only attached to the underling js
objects when their are active listeners on the Stream.
CHANGELOG : add notes about lazy events
_Event : small renaming
Merge pull request #45 from rmsmith/lazy_events

Lazy events