ruby19_stable_sort 0.0.1

The Ruby 1.8 Enumerable.sort_by method was stable, in that items with matching conditions were preserved in the same order. That's not the case in Ruby 1.9. Some of my code depended on this behavior, and I'm sure I'm not the only one. So here's a stable_sort_by method that preserves the orders of items with matching sort_by values.