countdown: let reminders be recurring

when creating a reminder, describe it as "and every X" and it'll refire
over that period, rather than only firing once

closes bss/dr.botzo#30

countdown/migrations/0004_countdownitem_recurring_period.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-02-24 01:06
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('countdown', '0003_auto_20170222_2025'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='countdownitem',
+            name='recurring_period',
+            field=models.CharField(default='', max_length=64),
+        ),
+    ]