Intializer Manager and DataFixture integration

BC Break: Yes
Doc PR: https://github.com/symfony-cmf/symfony-cmf-docs/issues/337

This PR fixes the issue about the initializer not being called when loading fixtures.

* New IntializerManager service which aggregates and executes Initializers
* Have placed a new PHPCRExecutor in this bundle - the old one is in `data-fixtures`

We can:

* Keep the PHPCRExecutor here (why not?) 
* Move this executor to `data-fixtures`
* (in data-fixtures) insert the EventManager into the Executors and have them fire an event after purging.

Console looks like this:

````bash
$ php app/console doctrine:phpcr:fixtures:load --no-interaction 
  > purging database
  > Executing initializer: Doctrine\Bundle\PHPCRBundle\Initializer\GenericInitializer
  > Executing initializer: Doctrine\Bundle\PHPCRBundle\Initializer\GenericInitializer
  > Executing initializer: DTL\WebBundle\Initializer\SiteInitializer
  > loading DTL\WebBundle\DataFixtures\PHPCR\LoadPageData
  > loading DTL\WebBundle\DataFixtures\PHPCR\LoadBlockData
  > loading DTL\WebBundle\DataFixtures\PHPCR\LoadPostData
````

I think we should also add a `getName` method to the initializers -- but maybe later (or rather in 1.1)