Merge lp://staging/~free.ekanayaka/storm/fix-order-by-expressions-in-ref-set into lp://staging/storm

The bug was introduced by the change in r358, which makes the order_by parameter of the ReferenceSet be processed by PropertyResolver. This branch tries to address the issue by making the PropertyResolver handle not only Storm classes, strings and Columns, but also expressions.

I think this a general solution that offers nice flexibility (you can write things like order_by=Desc("Foo.id"), unfortunately it's not complete because we'd need to handle all possible expressions which is not really nice. So all in all this feels something half-baked that will never be finished. If we think it's good enough, we can leave it like this, alternatively we can do something simpler but without the above flexibility (i.e. specify strings in sub-expressions). For example:

        elif isinstance(property, Expr):
            # Don't try to be fency here, just return whatever the user passed us
            return property

which would handle order_by=Desc(Foo.id) but not order_by=Desc("Foo.id").

Maybe the wise choice would be to go with this latter less powerful approach, and try to make it fancy only if use cases arise for it. OTOH it also feels that in practice you the most you want to do is indeed just a Desc() expression, and nothing more complicated, since you can do everything else with some application code using ReferenceSet.find. So maybe the solution of this branch is actually a compromise that makes sense.