Track RETRY events

[trifle]

Hi,

just an idea - in many cases, it's interesting to track the number of tasks that were retried.
That's especially useful to know when communicating with an external service.

Adding this should be as easy as changing this line:

https://github.com/tourbillonpy/tourbillon-celery/blob/master/tourbillon/celery/celery2.py#L46

from if task.state in ['SUCCESS', 'FAILURE'] and \
to if task.state in ['SUCCESS', 'FAILURE', 'RETRY'] and \.