Remember to return the state from dijkstra's algorithm when terminating ...

...early

(upon finding the shortest path to the node we are interested in).
github issue #76

When exiting Dijkstra's algorithm early by returning false from include_vertex! we return nothing, thereby contributing to the heat death of the universe and not much else.  Returning 'state' allows us to retrieve the shortest paths found so far.  This, I believe, is what @ehelle was getting at in issue #76.