bpo-29463: Add docstring field to some AST nodes. (#46)

* bpo-29463: Add docstring field to some AST nodes.

ClassDef, ModuleDef, FunctionDef, and AsyncFunctionDef has docstring
field for now.  It was first statement of there body.

* fix document.  thanks travis!

* doc fixes
diff --git a/Lib/test/test_opcodes.py b/Lib/test/test_opcodes.py
index 6806c61..8af098f 100644
--- a/Lib/test/test_opcodes.py
+++ b/Lib/test/test_opcodes.py
@@ -27,7 +27,7 @@
             with open(ann_module.__file__) as f:
                 txt = f.read()
             co = compile(txt, ann_module.__file__, 'exec')
-            self.assertEqual(co.co_firstlineno, 6)
+            self.assertEqual(co.co_firstlineno, 8)
         except OSError:
             pass