Add db_index to submit_date

Fixes #74

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_comments', '0002_update_user_email_field_length'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='comment',
+            name='submit_date',
+            field=models.DateTimeField(default=None, verbose_name='date/time submitted', db_index=True),
+            preserve_default=True,
+        ),
+    ]

@@ -62,7 +62,7 @@ class CommentAbstractModel(BaseCommentAbstractModel):
     comment = models.TextField(_('comment'), max_length=COMMENT_MAX_LENGTH)
 
     # Metadata about the comment
-    submit_date = models.DateTimeField(_('date/time submitted'), default=None)
+    submit_date = models.DateTimeField(_('date/time submitted'), default=None, db_index=True)
     ip_address = models.GenericIPAddressField(_('IP address'), unpack_ipv4=True, blank=True, null=True)
     is_public = models.BooleanField(_('is public'), default=True,
                                     help_text=_('Uncheck this box to make the comment effectively '