Initial Implementation of XML Format

[JemDay]

• I believe this is "spec compliant - I was wring this at the same time I was writing the spec.
• We may want to consider a streaming parsing implementation at some point, but I'm not sure that's needed now.
• Contains +ve and -ve tests along with some round-trip verifications.
• Implementation is contained and not visible outside the package/module.
• Comments welcome :-)

Closes #446