Fetch book information for given ISBN 10 or ISBN 13 number. Returns a map if result is found, nil if no result is found and :invalid_isbn if invalid.

> Openlibrary.Book.find_by_isbn("0812511816")
# %{ title: "The Eye of the World", authors: [%{}, %{}], ... }

> Openlibrary.Book.find_by_isbn("invalidisbn")
# :invalid_isbn

> Openlibrary.Book.find_by_isbn("isbn not present in db")
# nil