Changed Tempo initializer signature.

OLD: Tempo.__init__(*args).

NEW: Tempo.__init__(
    duration=None,
    units_per_minute=None,
    textual_indication=None,
    )

CHANGE: should be noted in the 2.15 change log (or equivalent).

CHANGE: note that text-only tempo indications like Tempo('Langsam') no longer
initialize; you must use Tempo(textual_indication='Langsam') instead.