Added a new DiGraph class which has very fast in-edge out-edge
operations. I will add other operations such as visitors and so on to
this later. The TestNewDigraph.cpp file creates a graph using two ways
and tests some of its components. I will move this to a CppUnit unit
test next - this was a very quick and dirty test, since I didn't want to
waste time before writing the interest code like Dijkstra's shortest
paths and so on.