Plague 3.15

Two major changes: 

1. My thanks to Michael Stone for speeding up the fire simulation - it is really fast now (it uses Numpy). Thanks Michael, you've made my day.
2. Plague keeps your score between games and the higher your score, the harder Plague becomes. Post your high scores on the blog (until I do something more clever - suggestions welcome).
   

Detailed changelog:

1. Michael Stone integrated NumPy into Plague leading to a very decent speed increase in cellular simulations – fire simulations are much quicker now, playable even. Thanks very much Michael!
2. incendiary grenades leave behind phosphorus that continues to burn after the initial flash of heat.
   
3.  guns dont automatically reload unless they are currently being used 
   
4. stopped guns from being drawn when they werent visible to the player 
   
5. made concussion grenade shockwaves hurt more and push less 
   
6. your score is conserved between battles 
   
7. the larger your score, the harder the game gets (more hives and more zombies) 
   
8. started adding leaders and followers