Include test for getNode method.
Use our own GraphvizGraph class to wrap the graphviz module's Graph class.
Fix bug that disallowed zero as a vertex ID.
Remove unused var in GraphvizGraph ctor.
Remove unused 'fs' require.
Fix bug where "use" property was not being hauled.

The "use" property must be mutable in order for 'output' to work.
Merge pull request #3 from mhfrantz/chore/Refactor-gremlin-graphviz-to-return-its-own-class_83678858

[Delivers #83678858] Refactor to return its own class