Since written books are quite different from empty books, the
visual clue that they are different items is really needed in-game.
I've added a "clasp" or "belt-with-buckle" like element to the
png from default_book, keeping them very similar but also
immediately recgnizably different.
I added the new texture to blockmen's license list since it's
obviously derivative of his work.
The PNG was run through a minimizer/optimizer to save space.
