Modified graphs.py, extended search.py

graphs.py function generate_rand_graph() now creates larger random graph
(20 nodes vs 10, 4 random link iterations vs 2)
search.py now has a return_path() method that finds a path in reverse
order, a find_path() method that reverses the return_path() result into
the proper order, and a format_path() method that prints the path out in
a visually pleasing fashion