Added Tremolo to indicatortools.

The class is currently a stub.

After implementation the class will be used like this:

    >>> chord = Chord("<c d>1")
    >>> tremolo = indicatortools.Tremolo(beam_count=2)
    >>> attach(tremolo, chord)
    >>> f(chord)
    \repeat tremolo 8 { c16 d }

This means that attaching a tremolo indicator to a chord will cause the chord
to behave differently at format time.

Attaching a tremolo to a note or rest will have no effect.