Add Cargo.toml

For the Cargo package manager.

The package manager does not allow for more than `[[lib]]` in `Cargo.toml`, so you'll have to figure something out for `sdl_image` and `sdl_mixer`. I'm guessing splitting them into separate repos is the way to go, but that's not my decision.

Also, I didn't know who should be included in `authors`, so I just put @brson for now