grouping.extras: new word group-by, like sql GROUP BY but is order-prese...

This new word works like itertools.groupby from Python: http://docs.python.org/3/library/itertools.html?highlight=itertools#itertools.groupby

There may already be some similar words in Factor to this one, but none of them is order-preserving which is unfortunate because that is very important in some contexts. Also added unit tests and docs for grouping.extras.