composer.json added for using Object_Freezer as dependency in other projects

I want to update https://github.com/clickalicious/Di to use modern PHPUnit features.

My first step would be to change Object_Freezer loaded by composer instead of a copied version in taht github repository. That's why I added a composer.json file to this project and configured a PSR-0 autoloader.

Also I have updated phpunit.xml.dist to use the generated autoload.php file as bootstrap to make use of the composer autoloader.