add natural order support

[evanchooly]

http://docs.mongodb.org/manual/reference/operator/natural/#op._S_natural

[trishagee]

This can be achieved at the moment using

Query.order("$natural")

and

Query.order("-$natural")

So I'm de-scoping it from the 0.108 milestone

[cstrempfer]

I could add an solution for that to pull request #1043. Should there be explicit methods for that, e.g. Sort#ascendingNatural(), or is adding a constant NATURAL to the Sort class enough? I prefer the first option, because the constant could easily be missed by developers.

[evanchooly]

I favor the explicit methods, too, though I wonder if naturalAscending() and naturalDescending() might be better. Either way is probably, fine, though. As for the package issue, I think a new Sort should be added outside the aggregation package. I'd use order rather than direction. The aggregation package's Sort would subclass the new one and cast order to int to preserve the existing API. Does that make sense?

[cstrempfer]

I'd use order rather than direction.

Do you talk about the package name? If yes, I move it to order. If not, is the utils package, ok?

The aggregation package's Sort would subclass the new one and cast order to int to preserve the existing API.

What's the order part? I cannot find an order class in the master branch. Do you mean creating a new enum for the direction?

[evanchooly]

Instead of Sort#direction, I'd name it Sort#order and the order field would need to be Object to accomodate -1, 1, and the natural ordering values. As for the package, I think it should be alongside Query itself probably or the package above.