13. Adding backend editing with sonata admin

[dbu]

With the frontend editing, you can not create new content. (This is a planned but not yet implemented feature.) This pull request adds sonata admins for the content.

As this again adds new bundles, you need to re-run composer

composer.phar install

If you installed from the last branch, you still need to clear the cache for symfony to pick up new translation files.

When you are logged in, you now see an "Admin" link in the toolbar.