SQLAlchemy Support

Hi, 
Thank you for your job! 
I added a class for working with my favorite ORM: SQLAlchemy.
````python
class UserFactory(SQLAlchemyModelFactory):
    FACTORY_FOR = User
    SESSION = DBSession

    email = 'commande.romain@gmail.com'
    username = 'rcommande'
````
The "SESSION" attribute is the SQLAlchemy session object with which you want to work.
http://docs.sqlalchemy.org/en/rel_0_8/orm/session.html