Ensure table's existence - as per the gludb spec.

def ensure_table(self, cls):
        """Ensure table's existence - as per the gludb spec."""
        cur = self._conn().cursor()

        table_name = cls.get_table_name()
        index_names = cls.index_names() or []

        cols = ['id text primary key', 'value text']
        for name in index_names:
            cols.append(name + ' text')

        cur.execute('create table if not exists %s (%s)' % (
            table_name,
            ','.join(cols)
        ))

        for name in index_names:
            cur.execute('create index if not exists %s on %s(%s)' % (
                table_name + '_' + name + '_idx',
                table_name,
                name
            ))

        self._conn().commit()
        cur.close()