Add options page to configure expiration time in hours

Hi!
Your plugin was exactly what I was looking for, but as many others I wasn't happy with the hard-coded expiration time of 48 hours. So I added an option page to configure the expiration time.
I tried to implement this according to your coding style, but I haven't localized my changes. And maybe there should be a little explanation text on the options page for the user.
Regards
Björn