events: define properties on prototype

Speeds up EventEmitter object construction by about 15-20%.

lib/events.js

@@ -37,6 +37,9 @@ function EventEmitter() {
 }
 exports.EventEmitter = EventEmitter;
 
+EventEmitter.prototype.domain = undefined;
+EventEmitter.prototype._events = undefined;
+EventEmitter.prototype._maxListeners = undefined;
 
 // By default EventEmitters will print a warning if more than 10 listeners are
 // added to it. This is a useful default which helps finding memory leaks.