方法说明：
为指定事件注册一个监听器。
语法：
emitter.on(event, listener) emitter.addListener(event, listener)
接收参数：
event (string) 事件类型
listener (function) 触发事件时的回调函数
例子：
server.on('connection', function (stream) {
console.log('someone connected!');
});
源码：
EventEmitter.prototype.addListener = function(type, listener) {
var m;
if (!util.isFunction(listener))
throw TypeError('listener must be a function');
if (!this._events)
this._events = {};
// To avoid recursion in the case that type === "newListener"! Before
// adding it to the listeners, first emit "newListener".
if (this._events.newListener)
this.emit('newListener', type,
util.isFunction(listener.listener) ?
listener.listener : listener);
if (!this._events[type])
// Optimize the case of one listener. Don't need the extra array object.
this._events[type] = listener;
else if (util.isObject(this._events[type]))
// If we've already got an array, just append.
this._events[type].push(listener);
else
// Adding the second element, need to change to array.
this._events[type] = [this._events[type], listener];
// Check for listener leak
if (util.isObject(this._events[type]) && !this._events[type].warned) {
var m;
if (!util.isUndefined(this._maxListeners)) {
m = this._maxListeners;
} else {
m = EventEmitter.defaultMaxListeners;
}
if (m && m > 0 && this._events[type].length > m) {
this._events[type].warned = true;
console.error('(node) warning: possible EventEmitter memory ' +
'leak detected. %d listeners added. ' +
'Use emitter.setMaxListeners() to increase limit.',
this._events[type].length);
console.trace();
}
}
return this;
};