Make timestamp optional

Because not all sources might offer one

django/website/data_layer/migrations/0002_auto_20150619_2132.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('data_layer', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='message',
+            name='timestamp',
+            field=models.DateTimeField(null=True),
+        ),
+    ]

django/website/data_layer/models.py

@@ -9,4 +9,4 @@ class DataLayerModel(models.Model):
 
 class Message(DataLayerModel):
     body = models.TextField()
-    timestamp = models.DateTimeField()
+    timestamp = models.DateTimeField(null=True)