[urdf_parser_py] Python3 compatibility
urdf_parser_py.urdf: Adding duck-typed parsing for ros_control and PR2 transmissions.
urdf_parser_py: Parsing new transmission format works. Need to add in test for old PR2 transmission types.
	test/calvin: Added in generated Calvin URDF provided by v4hn
	test/romeo: Reformatted to match Calvin test
urdf_parser_py: Added attribute preservation for xmlr.RawType (i.e., <gazebo reference="..."/>)
urdf_parser_py: Confirmed that both new transmissions are parseable in the same robot.
	test: Added in specific test for tranmsissions. Have it print the type.
	@todo: Make this a unittest.
Merge commit '763ab7206b2b631f5dd8ff584b007e719229d4b2' into feature/py-subtree