Laying some basic structure out for adding in CSV-read items
XML is almost certainly overkill for this.
Big, muddled commit:

* Removing ItemFactory, moving its functionality to data + GameStateManager
* Item has a more intuitve/flexibile constructor to be data-driven
* Others changed to use this new system
Moving items to GameStateManager. This commit has basis for rarity.

Need to get back to spawnable items by updating the GameWorldStage to use this.
Rarity needs to be in GameWorldStage, moving it there (1/2)
Rarity in GameWorldStage, as are a list of its items for easier spawning. Now to actually spawn again
Refactoring to make spawning/item setup more clear
Bugfixes to squash runtimes. Lots of "duh" moments. Still Runtiming, but error is in probability code now
Squashing more runtime errors. There's a null ptr still in the Rarity function, but that's expected. Work on that next.
Runtime errors squashed. A few bugs added, documenting in issues instead of commit
Merge pull request #28 from icbat/XML-and-more

CSV items, rarity algorithm