code: function(callback) {
      async.series([
        this.downloadPackageRootObjects.bind(this),
        this.downloadPackageVersions.bind(this)
      ], function(err) {
        return callback && callback(err);
      });
    }
docs: Download all the package root objects, package version objects,
  and tarballs.
  
  @param {Function} callback invoke when done.