Allow the use of Propel before Silex boots

As it is, it isn't possible to use Propel before Silex boots, which means it's not available during the bootstrap process aswell as when executing asserts (which was our problem originally).

```php
<?php

$app->get('/{slug}', array($this, 'function'))
    ->assert('slug', SlugPeer::assertUrl()) // assertUrl returns the list of authorized slugs separated by |
;
```