イベントリスナを追加・削除する（旧IE対応）。

add_event.js

var addEvent,
	removeEvent;

if (typeof window.addEventListener === 'function') {
	addEvent = function (el, type, fn) { el.addEventListener(type, fn, false); };
	removeEvent = function (el, type, fn) { el.removeEventListener(type, fn, false); };
} else if (typeof document.attachEvent === 'function') {
	addEvent = function (el, type, fn) { el.attachEvent('on' + type, fn); };
	removeEvent = function (el, type, fn) { el.detachEvent('on' + type, fn); };
} else {
	addEvent = function (el, type, fn) { el['on' + type] = fn; };
	removeEvent = function (el, type, fn) { el['on' + type] = null; };
}