Added Reps and Distance classes to node, support for them in the visitor
classes, and tests.

node.py - Added Reps, Distance
swim_visitor = added visit_reps, visit_distance methods
xml_visitor - added visit_reps, visit_distance methods
json_visitor - added visit_reps, visit_distance methods
test_ast_traversal - added TestXmlVisitor class and test_set_with_only_yards
method for xml testing
traversal.py - added json_builder method. It's a different attempt than the
visitor-based json_traverse method. Currently empty