Intiializer Manager and DataFixture integration

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.