New class: SQLSession

With the new class, SQLSession, you can store sessions in a database.
This class requires MultiSQL and by this offers support for all databases supported by MultiSQL.

Identification of the client is not only based on the session-id, but also on IP and user-agent, which should give a little more security.
IP- and user-agent will be treated with the database's escaping-function to prevent SQL-injections based on a faked user-agent.
For other values no escaping is done, you are responsible to ensure no SQL-injections are possible via values stored in the session.