Add support for back(String namedStep)

[Jotschi]

This would be very useful in order to efficiently retrieve the edge between two vertices

e.g.:

bothE().as("x").bothV().retain(relEntitiy).back("x")

http://gremlindocs.com/#recipes/finding-edges-between-vertices

---

Currently i would need to use a filter:

return inE("KNOWS").filter((TEdge edge) -> {
    return person.getId() == edge.outV().next().getId();
}).next(Knows.class);