Fixed doc tests for tuplet repr.
Improved FixedDurationTuplet repr.

Old:

    >>> scoretools.FixedDurationTuplet((2, 8), "c'8 d'8 e'8")
    FixedDurationTuplet((1, 4), "c'8 d'8 e'8")

New:

    >>> scoretools.FixedDurationTuplet((2, 8), "c'8 d'8 e'8")
    FixedDurationTuplet(Duration(1, 4), "c'8 d'8 e'8")

The new repr shows the duration of the tuplet explicitly.