Added a basic FindSDL2.cmake module.

I think it will only work on Unix based systems with pkg-config, but
it's soon to grow!
SDL2 is now being used in the CMakeLists.txt file.

That does mean though, that the target, ppm, can no longer be compiled.
Every cmake statement that would fail has been commented out...
Merge branch 'master' into SDL2-port
Organized the source tree. Separated a few files.
Fixed SDL2_INCLUDE_DIRS cache variable doc string.