openUrl(zipUrl, isBase64) {
    return utils_request(zipUrl, "binary").then(function (data) {
      return this.zip.loadAsync(data, {
        "base64": isBase64
      });
    }.bind(this));
  }
Load and Open an archive
@param  {string} zipUrl
@param  {boolean} [isBase64] tells JSZip if the input data is base64 encoded
@return {Promise} zipfile