For UPS tracking numbers, added a mod 10 checkdigit check.

Based on [this](http://answers.google.com/answers/threadview/id/207899.html), it appears that UPS tracking numbers aren't simply 18 characters long and begin with `1Z`.  There's a mod-10 check-digit which should be confirmed. This PR adds a `confirm` function, which, when present, must return true if the tracking number is truly legitimate.