adds catch_up configuration option

When a cluster hasn't run for a while, the default behavior is to keep executing schedules until they are caught up with the present time.
By setting `catch_up` to False, schedules will not play catch up and instead get rescheduled to the future.

django_q/conf.py

@@ -77,6 +77,10 @@ class Conf(object):
     # Global sync option to for debugging
     SYNC = conf.get('sync', False)
 
+    # If set to False the scheduler won't execute tasks in the past.
+    # Instead it will reschedule the next run in the future. Defaults to True.
+    CATCH_UP = conf.get('catch_up', True)
+
     # Use the secret key for package signing
     # Django itself should raise an error if it's not configured
     SECRET_KEY = settings.SECRET_KEY