Loot drop changes

Another small update.

Previously when you killed the enemies they always dropped the same diamond or potion. Now it is changed already a little bit and it will be futher devloped further on. The rook and pawn have 4 different diamonds (with a different 'score' each) to randomly pick from. The knight has 4 differend magic potions to drop. 

For now the bishop will always drop a small health potion, the queen will drop a potion upgrade and the king will drop a health upgrade.