Add threat evaluation

Give a bonus for each kind of attacked piece. Bonus
value is based on the type of attacked piece and the
type of attacking one.

Penalize pieces attacked by enemy pawns, also in
this case penality value depends on the type of
attacked piece.

This patch oboletes as redundant the increased mobility
count of the attcked squares that is then removed.

After 956 games at 1+0
Mod vs Orig  +262 =462 -232 51.57%  493.0/956 +11 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>