First roundtrip test case running with Hyperjaxb3

Today I've managed to execute the very first roundtrip test case with Hyperjaxb3. It's a primitive example with well-known purchase order schema, but it's a good starting point anyway. See tests/po.

In order to execute the test, you'll need to check out my changes to the JAXB 2 Maven plugin. Check out the Lexi2 branch from jaxb2-sources/jaxb-maven2-plugin and mvn install it. I hope we'll have these changes in the main branch (and maybe in the repository) soon, so it's a temporary inconvenience.