Ensure <cascade/> elements preceed <join-columns/>

When converting annotations based metadata to XML, the position of the <cascade/> elements resulted in XSD validation errors caused by the definitions of the association ComplexTypes:
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L463
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L482
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L495
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L517

Since changing the XSD might result in validation problems in existing mappings, changing the XmlExporter seems a better approach.

Note that the '<cache/>' element is not yet generated by the XmlExporter, but according to the XML Schema, it should precede the '<cascade/>' element.