Started implementing game management

Created Entity class for all game objects to inherit from.
Created Game class containing lists of templates to use when spawning
objects and a master list of Entities.

Added Pickup class to representpowerups and ammo.
Setup Ship, NPC, Projectile and Pickup as abstract classes inheriting
from Entity.  All classes have a TYPE enum.  We can create specific
objects of each of these by creating a child class of the appropriate
parent.

Added all classes to the Raiden namespace.