Make Image.unframe() return a new image, not a SubImage.

This is to make line and letter absolute positions relative to the page, not to the original framed image.

pixelocr/image.py

@@ -130,7 +130,7 @@ class Image(object):
         raise NotImplementedError
 
     def unframe(self, width=2):
-        return self[width:-width,width:-width]
+        return Image(self.data[width:-width,width:-width])
 
     def strip(self):
         """Strip top and bottom blank space.