Thriller!

Adds a rather... thrilling easter egg monster. Estimation for the easter egg spawn rates was:
1) ~100 sidewalked roads per average city
2) 8 cities per overmap
3) Round up to 1000 to account for other buildings, and to stay on the rare side.
4) x10 for number of overmaps

If you want to see how the monster acts I suggest debuging into a nearby city and spawning one in the middle of it.

Also cleans up the brain blob special to actually function by adding a new monster effect that prevents monsters from making their own plans (thus allowing their controller to control them well). When the controlling monster dies it wipes the effect away from all nearby controlled monsters.
Given what happens when you drop Thriller, I'd rate it diff 30 at least.