cs215/unit03/prob05_bridge_edges.py: implemented post_order()
cs215/unit03/prob05_bridge_edges.py: implemented number_of_descendants()
cs215/unit03/prob05_bridge_edges.py: implemented lowest_post_order()
cs215/unit03/prob05_bridge_edges.py: implemented highest_post_order()
cs215/unit03/prob05_bridge_edges.py: implemented bridge_edges(): gets into an infinite loop though :(