Adding two new objects:

* DocumentPathNode represents the path through the Avro + XML Schemas
when traversing either an XML document or an Avro datum.
* XmlToAvroPathCreator constructs the path through the XML Schema to
follow when converting an XML document to an Avro datum.
Flushed out DocumentPathNode, including equality checks on
SchemaStateMachineNodes (updated accordingly for equality checks).
Supporting the root element being part of a substitution group.