code: public FileListing listFiles(String prefix, String marker, int limit, String delimiter)
        throws QiniuException {
      return mBucketManager.listFiles(mBucketName, prefix, marker, limit, delimiter);
    }
docs: Lists object for Qiniu kodo.
  @param prefix prefix for bucket
  @param marker Marker returned the last time a file list was obtained
  @param limit Length limit for each iteration, Max. 1000
  @param delimiter Specifies a directory separator that lists all common prefixes (simulated
  listing directory effects). The default is an empty string
  @return result for list