[ResourceBundle][proposal/idea]Doctrine driver factory

I created a factory for the doctrine driver (defined in the `dependencyInjector` directory). Before this PR, a factory was 'simulated' in the `SyliusResourceExtension`. The driver used in `BaseExtension`  was hard coded too. The folder `Factory` was renamed to `Driver` because it was not really factories IMO. Spec are added too for each Driver.

@Sylius, is it a good idea (I hope!)?