Add a Symfony2 bundle to integrate with the Symfony2 Dependency Injection Container

By having the bootstrap managed by the DIC, it becomes a lot easier to use. Also, the XML file is way more maintainable than the annoying manual construction of the object graph.

Sample usage:

    $collectionBuilder = $container->get('simple_php_easy_plus.number_collection_builder');
    $engine = $container->get('simple_php_easy_plus.engine');
    $runner = $container->get('simple_php_easy_plus.runner');

    $collectionBuilder->add('1');
    $collectionBuilder->add('2');

    $calcul = new Calcul($engine, $collectionBuilder->resolve());
    $runner->run($calcul);

    $value = $calcul->getResult()->getValue();

This PR depends on #13.