Orchestra : Replace MongoDB by SQL and sqlalchemy

[davidfischer-ch]

To put a widely used ORM, sqlalchemy between our code and the database. We also keep MongoDB for Celery (workers) to save persistent states (e.g. revoked tasks ids) and the future units monitoring thread.

Maybe we can keep MongoDB as the main database, I need more investigations before choosing to keep or not to keep MongoDB.

Remarks

• Media's metadatas can be stored as XML DOM, thanks to a trick (see https://github.com/simplegeo/sqlalchemy/tree/master/examples/elementtree).