Tyr: add a family_type for each dataset

This way it's possible to find the lasts dataset loaded for each type od
data
Tyr: add a command for deleting old backup directories

For one instance:

    ./manage.py purge_instance fr-idf

For purging all the instances:

    ./manage.py purge_instances

The option "-b" can be use for run the task in background (with a
celery worker)
Tyr: give the ability to keep more than one dataset when purging

but the request is a lot less sexy than before :p
Merge pull request #547 from kinnou02/tyr_purge

Tyr: add a command for deleting old backup directories