fixed "importScripts" bug

Bug described here https://code.google.com/p/ie-web-worker/issues/detail?id=1


> The 'importScripts' method does not appear to be supported within worker js files.
>
> I tested by opening IE 10 and setting the browser and document mode to IE9 in the Developer tools setting. The console reports an error of `SCRIPT438: Object doesn't support property or method 'importScripts'`.