Add default Grid.iter_rows() implementation.

tailbone/grids/core.py

@@ -139,7 +139,13 @@ class Grid(Object):
         return self.fields.itervalues()
 
     def iter_rows(self):
-        raise NotImplementedError
+        """
+        Iterate over the grid rows.  The default implementation simply returns
+        an iterator over ``self.rows``; note however that by default there is
+        no such attribute.  You must either populate that, or overrirde this
+        method.
+        """
+        return iter(self.rows)
 
     def render(self, template='/grids/grid.mako', **kwargs):
         kwargs.setdefault('grid', self)