Implement #65: Use manager registry instead of document manager

As discussed in issue #65, I've changed the PHPCRBlockLoader so that it takes a `ManagerRegistry` on the constructor and provides a `setManagerName()` method to change this from "default".

**BC break**

Anyone using the manager name in the config will need to update:

    cmf_block:
        manager_name: <name>

(was `document_manager_name`)