Added small spiders to the crypt

part of #322

They are as simple as possible:
- They live for 10 turns
- Each central activespot from the crypt has 10% chances of spawning 1 per turn (which makes an average of 1 spider per central active spot for every turn)
- When they are outside the crypt (if the room is sold or destroyed), they die

With this logic, we don't have to handle anything like room absorbtion or active spot change