Y.Promise.EventNotifier: remove fulfilled promises from internal hash

[ItsAsbreuk]

Because promises that are bound to the notifier get into the internal _targets, they never get freed by the GC. Even fulfilled promises remain in the hash, leading to an increasing internal hash.

The issue is resolved by changing addEvents into this:

        addEvents: function (promise) {
            var targets = this._targets;
            targets.push(promise);
            promise.catch(
                function() {
                    return true;
                }
            ).then(
                function() {
                    targets.splice(targets.indexOf(promise), 1);
                }
            );
            return Notifier.decorate(promise);
        }

Ragerds,
Marco