#21 allow customizing Map types in DirectedSparseGraph

[Ilya Shlyakhter]

Currently, DirectedSparseGraph uses HashMaps for maps with vertex and edge keys.
I have specialized maps that work faster for my vertex and edge types.
It would be good if I could pass in a factory that DirectedSparseGraph would use to create maps -- both the initial maps for the 'vertices' and 'edges' fields,
and maps created by methods such as addVertex().
Another solution might be to add protected methods createVertexMap() and createEdgeMap().
Thanks!