Epidemics

Hello everyone,

Yes it's been nearly 2 years since I started doing this and with several attempts at assaulting 
the action queue, I present to you epidemics :scream:.

How the gameplay for epidemics works is detailed in:
https://github.com/CorsixTH/CorsixTH/wiki/How-epidemics-work

**UPDATE** 3/10/14 - Epidemics is now squashed to 3 commits, one for the entity map, one for the epidemic game logic etc and one for the levels using the new epidemic variables.