Proper usage of thresholding in minimax algo

	* Good moves are now considered "damn good", so
      they will be pruned by alpha-beta at upper tree level
    * Allowed 0-strength: IA will be dumb and chose only bad
      moves

Signed-off-by: sebhz <seb@zyglute.fr>
Allow for 0 seconds time

	* ... but the IA can still sneak a few us in between !

Signed-off-by: sebhz <seb@zyglute.fr>
Allow max exploration depth capping

Signed-off-by: sebhz <seb@zyglute.fr>
Added simple algorith to make IA weaker

Signed-off-by: sebhz <seb@zyglute.fr>