Implemented H2: Player Ladder Discs - Opponent Ladder Discs. 

ToDo: implement impact of blocked wins in H1, H2 calculation.

Changes: 
Added calculate method to calculate heuristics. Made individual
heuristic methods private (utility methods).

Added heuristic calculation for number of discs on a ladder that is not
blocked by opponents disc (for each point/potential ladder combination).
In addition implemented subtraction of opponents heuristic from total.