Added due dates for tasks in the data model.

Web form still needs to be updated.

attract/node_types/task.py

@@ -33,6 +33,10 @@ node_type_task = {
             }
         },
 
+        'due_date': {
+            'type': 'datetime',
+        },
+
         # For Gantt charts and the like.
         'time': {
             'type': 'dict',