SDL port

[DavidTruby]

For the following reasons, porting to SDL2 rather than SFML may be desireable:

• Platform support -- SDL2 supports the platforms SFML does, but additionally supports emscripten. This should allow the development of web games using tank and C++.
• SDL_main -- SDL provides a WinMain function for Windows, stopping a cmd.exe window popping up when the game is run there.
• General flexibility -- Due to being lower level, SDL will allow us to structure the engine in the way we want without fighting with the way SFML is structured.