Use the rollback() wrapper of class Persistent instead of manually calling the db4o rollback().
Future commits will need rollback hooks so it makes sense to have one central place where rollback() is called.
Allow overriding of override.properties "skip_tests=true" by commandline
"-Dskip_tests=false":
If we use "${property-name}" instead of "property-name" it will check
whether it evaluates to false. See
http://ant.apache.org/manual/properties.html#if+unless