schema listener for the dbal tables

according to https://github.com/symfony/symfony/issues/3560

fix #68

depends on https://github.com/jackalope/jackalope-doctrine-dbal/pull/139

note that i also change the doctrine dbal configuration a bit, we specify the connection **name** now, not the **service name** as before. i think its more consistent to specify the name, but we need to update the doc and sandbox. doc PR is https://github.com/symfony-cmf/symfony-cmf-docs/pull/188