编写一个 People 类，使其的实例具有事件绑定、触发事件、解除绑定功能。

Question

/*

 * TODO

 *

 */

People.prototype.sayHi = function () {

    console.log('Hi, I am ' + this.name)

}

// 目标效果：

var jerry = new People('Jerry')

jerry.sayHi() // => 输出：'Hi, I am Jerry'

jerry.on('greeting', say)

jerry.on('greeting', say2)

jerry.emit('greeting', 'Hi') // => 输出：'Hi, nice meeting you.' 和 'Hi, nice meeting you, too'

jerry.off('greeting', say)

jerry.emit('greeting', 'Hi') // => 只输出：'Hi, nice meeting you, too'

function say (greeting) {

  console.log(greeting + ', nice meeting you.')

}

function say2 (greeting) {

  console.log(greeting + ', nice meeting you, too')