Plague pathfinding

I added pathfinding to Plague last night (Eli Bendersky's python algorithm - thanks Eli), so survivors and zombies can be a whole lot smarter. It'll take a little while longer to re-engineer the AI to take advantage of that, and then I'll post a new release.

Plague 3.32

• Game mode system is now functional - allows the user to resume games (eg: after going back into the options menu). 
• Improved survivor graphics from Sean Lange.