History code rewrite (#1122)

Rearrange and rename all history heuristic code. Naming
is now based on chessprogramming.wikispaces.com conventions
and the relations among the various heuristics are now more
clear and consistent.

No functional change.

src/search.h

@@ -38,13 +38,13 @@ namespace Search {
 
 struct Stack {
   Move* pv;
-  CounterMoveStats* counterMoves;
+  PieceToHistory* history;
   int ply;
   Move currentMove;
   Move excludedMove;
   Move killers[2];
   Value staticEval;
-  int history;
+  int statScore;
   int moveCount;
 };