use Perl

I'm a couple of months late for mandatory ISBN-13 support, but I've uploaded Business::ISBN-2.00_01 so people who depend on the module can look at the new interface and make comments on it. It supports ISBN-13 seamlessy. Most of the internals are completely new: this was my second-ever module, and I did a lot of dumb things back then.

Interface changes (all open for discussion and improvement):

• I've renamed methods that used "ean" to use "isbn13".
• Things using the term "country" now say the more correct "group".
• The exportable functions are gone. They might come back, but as a separate module, like File::Spec::Functions.
• I got rid of the odd handling on constants at the user level. is_valid() is now just true or false. You can still get the specific parsing error with the new error().

This isn't ready for production. It's pretty good, but don't blindly replace what you have now. Give it a try, and if you run into a problem let me know.