Support for string.format in short messages in EyegradeException.
Several changes to the session database schema, plus schema versioning.

- The schema has now an explicit schema number that eyegrade checks
every time a session is opened.

- The schema allows exam alterations (revoke questions, change the
solution of questions, set an additional choice as correct). Present in
the schema for assuring future compatibility, but not yet implemented.

- The schema allows a session to have a title and description. Present in
the schema for assuring future compatibility, but not yet implemented.