Reorganization; see extended description.

I want to make a game engine, not a C++ wrapper for SDL. So all sources
that are related to the game engine library and which are not
necessarily 100% SDL related will go in the base of the src directory.

However, I am using SDL and I would like C++ wrappings. I don't want the
focus of the engine to be the wrapping though. So these will go in the
src/SDLCPP directory.

I need to figure out a way to structure the engine. To do that I am
rewriting SDL Snake (http://github.com/footballhead/sdl-snake) from C to
C++. These sources will go in the snake directory. When the project
matures a bit these sources may be split into a different GitHub repo.

I still need to figure out what to do with the tests directory.