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:.

I will update this later with more info about the commits.

**PLEASE NOTE**: If you are testing this there are a few things to consider:
* You can create an epidemic from the cheats menu or wait until a contagious player arrives (only later levels) 
* Multiple epidemics are supported -- created epidemics get added to a pool of future epidemics and one is chosen when appropriate to become the active epidemic - this means creating a new epidemic does not guarantee it will be the next one revealed to you!
* There are a lot of debug strings still in the console for reference.
* There is also another cheat which will toggle the infected icon over the infected players heads for all hidden epidemics **before** the epidemic is revealed - this is useful for tracking infected patients. 

This request needs a lot of testing for balance especially - again I will update with more details later about how things work exactly but feel free to test this out how it stands it should be pretty much there. 

**UPDATE** -- 19/01/14
We have moved development to my repository for now to keep the experimental changes separate and so wec an have our own tracker - find the repository here (https://github.com/sadger/CorsixTH/) and checkout the "experimental-epidemics" branch.

 If you find a bug or an issue with epidemics please can you put them on the issue tracker for sadger/CorsixTH https://github.com/sadger/CorsixTH/issues **not** upstream CorsixTH/CorsixTH that way we can track all the issues separately and they don't get lost in the main tracker as this feature is unreleased currently. 

**UPDATE** -- 24/9/14
This is ready to be integrated into the project. I've rebased and squashed what need squashing. I can still make changes now if anyone sees anything. 

How the gameplay for epidemics works is detailed in:
https://github.com/CorsixTH/CorsixTH/wiki/How-epidemics-work
Less math if this is kept in the same units as spread_factor. i.e. 200.