Changed dependence on KeyIndexableGraph to just Graph with some basic
guard code where needed.  Might need more; need to ask re
graph.getVertices(key, value).

Updated first set of unit tests to run tests through multiple graph
implementations. Currently makes use of TinkerGraph and MemorySailGraph;
might remove latter depending as it's quite different, although a decent
starting point since it does not implement KeyIndexableGraph.