Add support for sub-classing Graph, Vertex and Edge

[clue]

Using the current factory methods Graph::createVertex() and Vertex::createEdge() it's quite difficult to use e.g. an sub class of EdgeDirected there. We should actively encourage extending the base classes and perhaps at some point in time consider removing the create*() methods.

[clemens-tolboom]

+1 for support sub-classing.

But we should not remove the ability to attach data to either Graph, Vertex or Edge as #4 needs.