ReferenceByPathMarshaller that uses XPath instead of IDs

Would like to offer a third marshaller, ReferenceByPathMarshaller, to sit as best of both worlds between TreeMarshaller and ReferenceByIdMarshaller.

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TreeMarshaller:

• clean XML
• does not support duplicate or circular references

<people>
<person>
<car>
<make>Ford</make>
</car>
</person>
<person>
<car>
<make>Ford</make>
</car>
</person>
</people>

------------

ReferenceByIdMarshaller:

• adds bloat of IDs to XML
• supports duplicate and circular references (full object graphs)

<people id="1">
<person id="2">
<car id="3">
<make>Ford</make>
</car>
</person>
<person id="4">
<car reference="3"/>
</person>
</people>

------------

ReferenceByPathMarshaller

• clean XML... only writes references when needed
• supports duplicate and circular references (full object graphs)

<people>
<person>
<car>
<make>Ford</make>
</car>
</person>
<person>
<car path="../person[1]/car"/>
</person>
</people>