Allowing a user-defined type (Avro Schema, GraphVis Node Identifier,
anything) to be set on an XmlSchemaDocumentNode.

Also started work on AvroSchemaApplier to set the Avro Schema on
XmlSchemaDocumentNodes as required.
Adding unit test for AvroSchemaApplier.  Could be flushed out more, as
could all my other unit tests.