Adds (tested) Graph and Edge classes

This has the Graph and Edge classes.  As far as I know everything's working, but it's possible I added extra quotation marks to the digraph output, and I can't test that on this computer.

There's some commented out code in Graph.java that uses a Vertex class.  I originally wrote and used one of those, but it was basically just a wrapper for the filename, so I just made Graph use the names instead.  I can upload the Vertex.java if it becomes necessary though.