def add(pendable, lifetime=None):
    """Create a new entry in the pending database, returning a token.

        :param pendable: The IPendable instance to add.
        :param lifetime: The amount of time, as a `datetime.timedelta` that
            the pended item should remain in the database.  When None is
            given, a system default maximum lifetime is used.
        :return: A token string for inclusion in urls and email confirmations.
        """
