Extended Tempo.__div__().

You can now divide tempo objects by numbers:

    >>> Tempo(Duration(1, 4), 60) / 2
    Tempo(duration=Duration(1, 4), units_per_minute=30)

And you can still divide tempo objects by other tempo objects:

    >>> Tempo(Duration(1, 4), 60) / Tempo(Duration(1, 4), 45)
    Multiplier(4, 3)