Reorder #includes

Trying to follow the style in Google C++ style guide:
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Names_and_Order_of_Includes
Put image loading/caching into a separate class

Created class Graphics::Images in images.cpp.

Also removed SDL.h include from game.h.  Using uint32_t there instead of
SDL's Uint32.