Merged in urdf_parser_py

Didn't look a like a filter-branch or subtree was necessary, so just did a basic merge: https://github.com/eacousineau/urdfdom/commit/0cc7489891c418f225edda6e6ee092a22b9c4947

Addressing https://github.com/ros/urdfdom/issues/4#issuecomment-23557044

Should we rename `xml_reflection` to something a little more specific to python?