Added SegmentMaker baseclass to experimental segmentmakertools package.

The class is currently a stub.

All segment-makers should inherit from SegmentMaker analogously to the way that
all rhythm-makers inherit from RhythmMaker.

PACKAGE STRUCTURE: we'll probably leave segmentmakertools in experimental for a
couple of point releases before moving to mainline. Seems like a couple of
complete scores should be done with segment-makers before making public.