Cleaned up a few objects in core package.

1. Deleted PinCountSum — moved code to PinCount;
2. Modified score() in Frame, PinCount, Score, Ball,  ScoredBall. Made
Score.add() private, instead of public.
Made method access modifiers more restrictive.

In core package objects (Ball, Frame, PinCount, Score), as well as in
ScoredBall, changed method access modifiers to package or protected
access.
double dispatch to remove long parameter passing.

double dispatch between Frame and Score used to get rid off Score
passed from Bowling —> Frame —> PinCount.