fix for Firefox : "An attempt was made to use an object that is not, or is no longer, usable" for loader of text.
fix loaders to run on Firefox (responseType should be set after open(...) )

May be loaders could be simplified with code like :

    var f = HttpRequest.request(assetRequest.URL,  method : 'GET', responseType : 'arraybuffer');
    return f.then((httpRequest) {
      return (httpRequest.status == 200) ? httpRequest.response : null;
    });

and failed instead of "return" null if error ?
Merge pull request #13 from davidB/master

fixes loaders for Firefox