Potential starting point

With this initial round of exploration with Jaxb, I think we have a solid starting point for implementing the API.
Main changes / additions:
 - schema now allows arbitrarily ordered elements (ugly aOrBOrC constructs are split into  `List<a,b,c>`)
 - Objects created by Jaxb can be freely "extended" (se the _src/main/java/extended_)
 -  Visitor skeletons for the parsed tree are automatically generated