Issue #22. Some additional logging.
Issue #23. Test project.
Issue #23. Added hasCatalogs() method to check if catalogs are set.
Issue #23. Null log.
Issue #23. Minor changes.
Issue #23. Added a check for simultaneous usage of strict mode and
catalogs.
Issue #23. If the resolver can't resolve, it should return null, not
systemId.
Issue #23. Added a test project.
Issue #23. `<strict>true</strict>` by default not to break
backwards-compatibility.