Remove action_counter from message and action implementation.
Update testing infrastructure to new task_level information.
Fix Twisted and Python 3 breakage.
Get rid of remaining references to action_counter.
News entry.
Better documentation of ``task_level``.
Address review comment: Fix docs.
Address review comment: Coding standard.
Address review comment: use .format().
Merge pull request #127 from ClusterHQ/message-ordering-126

Change fields to allow calculating consistent message ordering.

Fixes #126.