Let NamedGraph implement Graph interface

Use the WrappedGraph class or the WrapperGraph interface. https://github.com/tinkerpop/blueprints/blob/master/blueprints-core/src/main/java/com/tinkerpop/blueprints/util/wrappers/wrapped/WrappedGraph.java https://github.com/tinkerpop/blueprints/blob/master/blueprints-core/src/main/java/com/tinkerpop/blueprints/util/wrappers/WrapperGraph.java