def login(credentials, identifier_name=None):
    """ -> (identity, headers)
        
        o This is an API for browser-based application login forms.
        
        o If 'identifier_name' is passed, use it to look up the identifier;
          othewise, use the first configured identifier.

        o Attempt to authenticate 'credentials' as though the identifier
          had extracted them.
          
        o On success, 'identity' will be authenticated mapping, and 'headers'
          will be "remember" headers.

        o On failure, 'identity' will be None, and response_headers will be
          "forget" headers.
        """
