Holiday time

I'm going on holiday for 8 days, so I won't be updating Plague during that time - this build seems to be bug free and is pretty fun (no one has complained yet...) so it seems like a good place to leave it for a week or two. 

I will however be reading up on NumPy, with a view to using it for my fire and explosion algorithms when I get back. Hopefully this will alleviate the worst CPU hog. After thats done it'll be time for adding lots of new fun stuff!