pieceScore: tidy up
ai: improve board eval

Add bonus for two bishops and penalties for knights at endgame. Also,
fix comments and tidy up the constants in a single file. Furthermore,
hit myself in the head for not splitting this into several commits.
ai/board: move piece scores to board.h and add SIDE_SCORE

SIDE_SCORE represents the starting score of a side. Used to interpolate
and scale other scores without magic numbers
ai: boardEval: king safety

Take king safety into account. Only consider the pawn shield for now.
ai: add 11n heuristic

Misterious name... what could it mean...?
ai: add command line option for h11n heuristic
ai: fix comments