Default jobtastic tasks to track_started = True

[winhamwr]

Celery has a built-in track_started option. For Jobtastic tasks, change the default from False to True, so that we can let the user know that their task has begun executing and that we're estimating the remaining time.

• Ignore CELERY_TRACK_STARTED and document that fact
• Document this as the default
• Update jquery-celery to know about the STARTED state.

This removes the need to call self.update_progress(0, 0).

[winhamwr]

cc @kylegibson I think this is better than putting in an automatic self.update_progress call. It also means that we could change the time estimation to treat the first update_progress call as the beginning of time, which would give more accurate progress estimate for tasks that have a fixed-ish amount of up-front work, followed by a more incremental list of things to do. That's actually a lot closer to what we see, at least.