Add composer.json

This should fix #19.

Once this is merged, people should be allowed to include this package with the [VCS config](https://getcomposer.org/doc/05-repositories.md#loading-a-package-from-a-vcs-repository).

Ideally, you could then [submit](https://packagist.org/packages/submit) the package to Packagist (the Composer central repo), so people don't need to use the VCS config (more informations [here](https://packagist.org/about)). That's not something I can do, as it requires the repo's maintainer to open an account to submit the package.