Change all uses of sort.Sort to sort.Stable.

Using sort.Stable ensures that even if the sort keys are the same, the order of the sort will not randomly change.  Using the old sort.Sort, if you had pages with no date, the lists of those pages would randomly reorder every time you regenerate the list, causing spurious changes to the output.  Now they'll always get ordered in the same way.  This does require Go 1.2.1, but I think we already require at least that.