Created a TraverserGenerator interface. Now steps,traversals,etc. no longer need to know about Traverser types. They get them for a registered generator. This is staging for a few thing -- a suite of traverser types as OLAP can have various optimization not available to OLTP (Refs #200), the ability to have SackTraverser (Refs #330), the ability for DSL creators to have their own Traversers that are specific to their needs (Refs #47), support for traverser conversion mid-stream (Refs #331).