Fix and improve Statement.__doc__

- The type of `set_vars` and `used_vars` were wrong.
- ":param ...:" was used instead of ":type ...:".
- The parameter table is aligned to make it easier to read.
Use literal block instead of >>> for code example
Document pr.Simple.__init__
Remove a philosophical (and confusing) statement
Document pr.SubModule
Add a very short tutorial on parsing_representation
Document Scope.get_defined_names
Document evaluate.get_defined_names_for_position
Document get_names_of_scope with examples
Make doctest Python 3.x compatible
Merge pull request #148 from tkf/docstring-improvements

Improve docstrings