#6 Add ability to sort by related object to pxdb_collection

[Sandy Smith]

It will take a little thinking, but there's no reason I
can see that we can't imitate the way that picktable
values are able to be sorted (through the set_orderby()
method).

I think it would use the same signature as the
picktable sort, by default sorting by id, and if the
related_cols array is passed, sort on those fields. It
would also probably be left joined.