Added the Action typedef.  Minor formatting and comment corrections.
Added a UML class diagram of the SceneNode class.
Added the redistributables for chapters 1-6.  This includes both debug and
release DLLs, textures, and a font.  Also included the .gitignore file
which no longer ignores the Redist directory.
Chapter 7 - added enemy aircraft w/ simple AI, projectiles (machine gun and
missiles), pickups (health refill, missile refill, fire spread, and
fire rate), collision detection between nodes of the SceneGraph and their
responses, and the world's update cycle automatic removal of entities.