code: def dir(self, folder='', prefix=''):
          
          files, folders = self.ls(folder)
          result = files
          inner = []
          for fld in folders:
              try:
                  inner += self.dir(folder + '/' + fld, prefix + fld + '/')
              except:
                  pass
          result += inner
          if prefix:
              result = map(lambda a: prefix + a, result)
          return result
docs: Lists all the files on the folder given as parameter.
          FtpHandler.dir() lists all the files on the server.
  
          :para folder: the folder to be listed.
          :type folder: string
  
          :param prefix: it does not belong to the interface,
                         it is used to recursively list the subfolders.
  
          :returns: a list with all the files in the server.