func (db *Database) Login(user, pass string) error {
	return db.Session.Login(&Credential{Username: user, Password: pass, Source: db.Name})
}
func (db *Database) Login(user, pass string) error {
	return db.Session.Login(&Credential{Username: user, Password: pass, Source: db.Name})
}
// Login authenticates with MongoDB using the provided credential.  The
// authentication is valid for the whole session and will stay valid until
// Logout is explicitly called for the same database, or the session is
// closed.