Add composer.json for composer tool (http://getcomposer.org/).

Please merge this request to allow using your library with composer dependency manager (http://getcomposer.org/).
Additionally please register at  https://packagist.org/ and submit your library via https://packagist.org/packages/submit. It will allow to keep your package up-to-date in Packagist.
It would be great if you could add a tag to this repo after merging (it will be marked as a stable release on Packagist).