Add composer.json file

I'd like to include this library in one of my projects, via Composer. Therefore, I need it to have its own `composer.json` file. This enables you to add this repository as a dependency to any project.

Also added `PSR-0` autoloading on the `classes` folder as that should ideally just be autoloaded. It's just classes in there that don't include each other, right? If not, we should make that happen. :smile: