modified Prisoners' Dilemma - higher payoffs are better.

also changed Leaderbord to work correctly without reversal

New payoffs are:
a = 2, b = 0, c = 3, d = 1.
These fulfil both requirements:
1) c > a > d > b  (wiki: T>R>P>S)
2) 2*a > c + b  (wiki: 2*R > T+S)