- Add escape mechanism for environment variables ($$ or \$ ?)?
- Make it possible to store current configuration?
- Add set_base_path?
- Allow write access to configuration?
- Add a FileTouchListener interface?
- Are the interfaces of get_parameters and get_parameter_array
  good enough?
- Restructure test cases, so they will run faster.
- Measure performance.

- Allow initial XSL transformation? No.
- Support different XML parsers / XPath engines? No.
- Add some caching? No.
- Consider text nodes in get_parameters? No.
- Make default value for empty elements in get_parameters
  configurable? No.
- Add method print(destination) to print the current configuration
  to destination? Or better implement to_s? No.
- Add get_path and set_base_dir? No.
