Make GPCFieldRenderer display check digit separate from main barcode data.

It is presumed that this is a good idea in general; I guess we'll see.

tailbone/forms/renderers/products.py

@@ -20,7 +20,6 @@
 #  along with Rattail.  If not, see <http://www.gnu.org/licenses/>.
 #
 ################################################################################
-
 """
 Product Field Renderers
 """
@@ -64,6 +63,13 @@ class GPCFieldRenderer(TextFieldRenderer):
         # Hm, should maybe consider hard-coding this...?
         return len(str(GPC(0)))
 
+    def render_readonly(self, **kwargs):
+        gpc = self.raw_value
+        if gpc is None:
+            return ''
+        gpc = unicode(gpc)
+        return '{0}-{1}'.format(gpc[:-1], gpc[-1])
+
 
 class BrandFieldRenderer(AutocompleteFieldRenderer):
     """