switch to PSR-4

Update to PSR-4 autoloading as discussed in symfony-cmf/symfony-cmf#185.
we did not ignore this on purpose. people creating a project starting from this should start to track their composer.lock. we just have to take care to never commit it here in the template project. see also https://github.com/symfony/symfony-standard/blob/master/.gitignore where they do the same.