due_today.pl, now with more correctness

OK, apparently I need some date lessons, because 4/31/2009 isn't a real date (go figure). Turns out Add_Delta_Days requires the date from localtime to be converted prior to being used. Academic error, but unfortunately one that can slip through the cracks until a certain day (5/31/2009) shows up, and makes things go boom.

So, here's the lates rev of due_today.pl, which will show you items in your todo.txt file that are due today, past due, and upcoming.