Add TestDocstring
Make pr.Statement "document-able"
Add attribute docstrings (PEP 257) support
Define Name.docstr as a proxy for Statement.docstr
Add test_attribute_docstring
Unconditionally set attribute docstrings

As Statement.assignment_details triggers parsing inside
statement let's don't check assignment_details when setting
attribute docstrings.
Include statements to Script.definition if it has docstring
Check the case where multiple docstrings are defined
Merge pull request #141 from tkf/attribute-docstring

Attribute docstring support