I'd rather not introduce a new name -- and I'm not real fond of `sortBy(f)` since `f` is not a comparator, even though the method name seems to imply that it is.  I think I'd rather see a `sortByKey(f)`, but Ill have to think about how to do that while maintaining the proper implicit conversions among `RDD`, `PairRDDFunctions` and `OrderedRDDFunctions`.