0.9.0

Added reversePostOrder to DirectedGraphPlugin. This will allow a DirectedGraph to perform a
closure on each vertex in reverse post order (topological order). Also added reversePostOrderSort which
returns the vertex names in reverse post order of a graph.

DirectedGraphPlugin was reorganized making it possible to javadoc methods dynamically added to the graph.
The methods added are static in DirectedGraphPlugin. They are added to Graph's metaClass using method pointers while currying the graph param to the apply method. This could be used in future plugins.