Include SFML source as well

[Jebbs]

Been brainstorming on how to make building the library more simple and to make sure things are built more correctly, I have decided that I want to include the SFML sources with this project. This should help with a couple of things:

• The build tutorial can use SFML's build tutorial directly as it will be the exact same process.
• Only one set of sources will need to be downloaded.
• Only one library will need to be built.
• Since SFML is kept more up to date with its CMake stuff, it will be better to base DSFML-C's CMake stuff off of that instead.

[Jebbs]

The merge took place today, so everything is working as I wanted it too. (as far as I can tell)