Added child version cascade tests to 15_Versioning

Tests to demonstrate how I believe version cascade should work. 

For now, I've added a child node to the 'versioned' node in the fixtures and added extra test cases to 15_Versioning/VersionTest.php. However, if it's better I'll be happy to move these tests to a separate test class (and with separate fixtures if needed).

Also, does the CND type look correct?

    [phpcr:versionCascade] > nt:unstructured
        + * multiple copy