data-grid-app: added universal function for extracting ordering.

- now supports format a.b
- supports passing a ref column e.g User.username, get's handy for joined querie

rhodecode/apps/_base/__init__.py

@@ -20,6 +20,7 b''
 
 import time
 import logging
+import operator
 
 from pyramid.httpexceptions import HTTPFound
 
@@ -283,6 +284,15 b' class DataGridAppView(object):'
         draw = safe_int(request.GET.get('draw'))
         return draw, start, length
 
+    def _get_order_col(self, order_by, model):
+        if isinstance(order_by, basestring):
+            try:
+                return operator.attrgetter(order_by)(model)
+            except AttributeError:
+                return None
+        else:
+            return order_by
+
 
 class BaseReferencesView(RepoAppView):
     """