Add manage.py cleanlinks command to remove expired links, add sample

cron script and notes on setup in README.md.

README.md

@@ -42,3 +42,11 @@ $ pip install -r requirements.txt
 ```
 
 and you should be good to go.
+
+### Cron
+
+In order to expire links, you need to set up a cron job to run the manage.py
+expirelinks command regularly. A sample cron script is available in the
+repository and is named go-cleanlinks.cron. Drop this in cron.hourly and
+change the paths so that they point to the virtualenv activate script and
+manage.py.

go-cleanlinks.cron

+#!/bin/sh
+# Cron job to automatically expire outdated links, put this in cron.hourly
+ACTIVATE_PATH=/path/to/virtualenv/activate
+MANAGE_PATH=/path/to/go/manage.py
+
+source ${ACTIVATE_PATH}
+python ${MANAGE_PATH} expirelinks

go/go/management/commands/expirelinks.py

+from django.core.management.base import BaseCommand
+from django.utils import timezone
+from go.models import URL
+
+
+class Command(BaseCommand):
+    help = 'Removes expired links from the database'
+
+    def handle(self, *args, **options):
+        for toexpire in URL.objects.filter(expires__lte=timezone.now()):
+            self.stdout.write("Removed URL %s (%s), expires %s" % (toexpire.target, toexpire.short, toexpire.expires))
+            toexpire.delete()