make backoff setting configurable

clacksme/clacksme.py

@@ -84,7 +84,14 @@ class MailboxNotifier(object):
                     "event_id": uid})
                 if self.last_check is not None:
                     delta = now - self.last_check
-                    if delta.seconds > 60:
+
+                    try:
+                        backoff = System.get(System.key == "backoff")
+                    except System.DoesNotExist:
+                        backoff = System(key="backoff", value="60")
+                        backoff.save()
+
+                    if delta.seconds > int(backoff.value):
                         notify = True
                 else:
                     notify = True