- to check gevent.Event API conformance

- this makes Event API compatible with threading.Event
- compatibility with Queue.put is not need, because Greenlet does not threat put/put_exception
  as special method names the way Proc did it
- put and put_exception still available but raise deprecation warnings

- start event is stored as an attribute; it is cancelled by throw()
- spawn_later returns Greenlet instace, not tuple (greenlet, event)
- _run / args / kwargs attributes are removed when Greenlet finishes execution