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.

docs/tasks.rst

@@ -54,6 +54,10 @@ timeout
 """""""
 Overrides the cluster's timeout setting for this task.
 
+ack_failure
+"""""""""""
+Overrides the cluster's :ref:`ack_failures` setting for this task.
+
 sync
 """"
 Simulates a task execution synchronously. Useful for testing.
@@ -244,6 +248,7 @@ Reference
    :param str group: An optional group identifier
    :param int timeout: Overrides global cluster :ref:`timeout`.
    :param bool save: Overrides global save setting for this task.
+   :param bool ack_failure: Overrides the global :ref:`ack_failures` setting for this task.
    :param bool sync: If set to True, async will simulate a task execution
    :param cached: Output the result to the cache backend. Bool or timeout in seconds
    :param broker: Optional broker connection from :func:`brokers.get_broker`