Add composer.json for composer support (https://getcomposer.org/)

For a basic composer support, please add this composer.json file to your repository.

Maybe you could register this package on https://packagist.org/.

Right now I added the autoload files feature, maybe I find some time for a PR with added [PSR-4](http://www.php-fig.org/psr/psr-4/) compability.