Cleaned up and refactored MotherBrain
Refactored draw/render method.
Renamed draw to render in Displayable interface.
Refactored method declarations to throw NotImplemented exceptions.
Renamed Entity to abstract entity.
Added Collidable interface.
Refactored State interface to use abstract class.
Improved GameStateStack by using and eliminating generics where appropriate.
Used diamond notation and removed unused variables.
Made game state stack private and final.
Removed superfluous override.
Refactored resource loading of images and tiled file.
Brought down visibility for fields and added setters for colliding and evaluating.
Encapsulating variables.
Enumerated status code used by System#exit()
Fixed verbiage in Javadoc
Combined authors.txt into NOTICE.md and added Slick2D license.
Fixed Javadoc method reference parameters.
Added more than default template for Javadoc.
Removed unused import.