Make sketchbook strip should handle non zero padded number.

[wholewheattoast]

If a sb has 9.jpg, then 10.jpg, and later 100.jpg it should appropriately handle for ordering.

There are already url's for older books, if possible let's not break them.

[wholewheattoast]

Script now sorts like a human, and has special casing for 'back cover' and 'ifc'