NPC Fixes

Moved the unnecessary NPC code from GameEngine to NPC, put it into
functions that can be called in GameEngine. Chris, I didn't change any
of your file path names so I think it should probably work. John, you
will have to change the path names (sorry) and Thomas you'll have to
change the "SDL2/SDL" includes to just "SDL". But aside from that, this
should compile. Let me know if it doesn't