Replace StreamFactory with IteratorUtils in io.
Refactor list() method in IteratorUtils to use fill()
Introduce IteratorUtils.collectMap function.

Get rid of more StreamFactory usage in IO.
Add IteratorUtils.groupBy function

All StreamFactory methods are removed from GraphSONModule.
Remove the groovy sugar for save/loadGraph now that Graph.io() is done #376

Updated docs to remove groovy sugar references to save/loadGraph.