three isbns

So, the normal case with multiple isbns is that a book has "equivalent" ISBN-10 and ISBN-13 entries, but I have come across a case that doesn't fit that. There are three, a US isbn10, a US isbn13, and a UK isbn13. With it only being possible to store one canonical isbn13 number in valid_isbn, I simply picked the first one.

In the future a more extensible scheme will be needed to allow any number of "numbers" of different types - isbns, issue #s, vol #s, barcodes ... probably others.