- Implement to_hash. It should behave exactly like Perl's
  XML::Simple.
- Finalize set_parameter and allow creation of new nodes.
- Add escape mechanism for environment variables ($$ or \$ ?)?
- Are the interfaces and semantics of get_parameters and
  get_string_array good enough?
- 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.
