MUI 监听返回（mui.back）按2次返回退出app

mui.init({
	keyEventBind: {
		backbutton: true //开启back按键监听
	}
});

var clickNum = 0;
mui.back = function(event) {
	clickNum++;
	if(clickNum > 1) {
		plus.runtime.quit();
	} else {
		mui.toast("再按一次退出应用");
	}
	setTimeout(function() {
		clickNum = 0
	}, 2000);
	return false;
}