Add option for acknowledging failed tasks (globally and per-task)

If a task fails with an exception, it is retried until it
succeeds. This is contrary to what is said in the documentation: under
the "Architecture" section, heading "Broker" it says that even when a
task errors, it's still considered a successful delivery. Failed tasks
never get acknowledged however, thereby being retried after the
timeout period. See also issues #238 and #194.

This patch adds an option to acknowledge failures, thereby closing
issue #238. Issue #194 would require some more work. The default of
this option is set to `False`, thereby maintaining backwards
compatibility.

django_q/conf.py

@@ -110,6 +110,11 @@ class Conf(object):
     # Number of seconds to wait for a worker to finish.
     TIMEOUT = conf.get('timeout', None)
 
+    # Whether to acknowledge unsuccessful tasks.
+    # This causes failed tasks to be considered delivered, thereby removing them from
+    # the task queue. Defaults to False.
+    ACK_FAILURES = conf.get('ack_failures', False)
+
     # Number of seconds to wait for acknowledgement before retrying a task
     # Only works with brokers that guarantee delivery. Defaults to 60 seconds.
     RETRY = conf.get('retry', 60)