[Web4lib] Conversion between ISBN-10 and ISBN-13

[Tim Spalding]

Minor point: Bookland EANs are the same as ISBN 13s now, which is why
you can convert in both directions without problem now. But this won't
always be so. Once they start using 979 numbers, conversion won't make
sense. You can do it mathematically, however, so I'm guessing a lot of
valid but nonsensical ISBNs end up here and there on the web.

Someone should offer single and batch converstion as a free API, not
an online form and an offer to have a "representive" call you for
larger jobs.

Does anyone want that, or shall I?