Add a separate image for a written book.

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.

mods/default/README.txt

@@ -148,6 +148,9 @@ BlockMen (CC BY-SA 3.0):
   heart.png
   gui_*.png
 
+sofar (CC BY-SA 3.0):
+  default_book_written.png, based on default_book.png
+
 Neuromancer (CC BY-SA 2.0):
   default_cobble.png, based on texture by Brane praefect
   default_mossycobble.png, based on texture by Brane praefect