检测浏览器类型
1. var userAgent = navigator.userAgent.toLowerCase();
2. var is_opera = userAgent.indexOf('opera') != -1 && opera.version();
3. var is_moz = (navigator.product == 'Gecko') && userAgent.substr(userAgent.indexOf('firefox') + 8, 3);
4. var is_ie = (userAgent.indexOf('msie') != -1 && !is_opera) && userAgent.substr(userAgent.indexOf('msie') + 5, 3);
5.var is_mac = userAgent.indexOf('mac') != -1