Check & document xmldb-api version/origin

The currently includetd xmldb-api (xmldb.jar) is of unknown version, but supports xquery (that is missing in many maven-provided official releases of the api)

TODO: Check if the version available at http://xmldb.exist-db.org/ can replace the unknown version in the EEE repo-directory. If so change and document it. Or even better: find official Maven repo for it.
