Logbook stream should print header if no records

[bje-]

Even if there are no records yet, calling stream() on the logbook should print just the header. I've described it by updating the doctest to show what I mean:

            >>> log = Logbook()
            >>> print log.stream  # doctest: +NORMALIZE_WHITESPACE
            gen
            >>> log.append({'gen' : 0})
            >>> print log.stream  # doctest: +NORMALIZE_WHITESPACE
            0
            >>> log.append({'gen' : 1})
            >>> print log.stream  # doctest: +NORMALIZE_WHITESPACE
            1

This is important when it takes a while for the results of generation 0 to appear.