- for hackers, refactored the "visitor" system of ClauseElement and

- for hackers, refactored the "visitor" system of ClauseElement and
SchemaItem so that the traversal of items is controlled by the
ClauseVisitor itself, using the method visitor.traverse(item).
accept_visitor() methods can still be called directly but will
not do any traversal of child items.  ClauseElement/SchemaItem now
have a configurable get_children() method to return the collection
of child elements for each parent object. This allows the full
traversal of items to be clear and unambiguous (as well as loggable),
with an easy method of limiting a traversal (just pass flags which
are picked up by appropriate get_children() methods). [ticket:501]
- accept_schema_visitor() methods removed, replaced with
get_children(schema_visitor=True)
- various docstring/changelog cleanup/reformatting