Externalize and make explicit DB session management, introducing user ID.

Session management is pushed up to the user with this commit. This allows for lazy loading of attributes and relations, since the session will be handler-wise (and not operation-wise).

The new primary key for users is a numeric ID, which offers better perfomances and will allow for more privacy in cookies (no username inside). This second change is closely coupled to the one above and is contained in file `bitsd/persistence/model.py`.