More documentation test items.
Simplify do_help.
Simplify traverse_help.
Refactor get_help.

- Don't squash AttributeError exceptions from help_ methods.
- Return the top-level docstring for the Shellac class.
- Move help_help into the docstring of do_help
- Allow lower-level help_ methods and do_ docstrings to be empty which reverts
  to higher-level results
- Allow a help_ method without a do_ method.