docs: def mkdir(self, foldername):
          'Create a new subfolder and return the new JFSFolder'
          #url = '%s?mkDir=true' % posixpath.join(self.path, foldername)
          url = posixpath.join(self.path, foldername)
          params = {'mkDir':'true'}
          r = self.jfs.post(url, params)
          self.sync()
          return r