Add name-based access to ResultSet columns with operator[]

[mrts]

    dbc::PreparedStatement::ptr select =
        _db.prepareStatement("SELECT * FROM person ORDER BY name");
    dbc::ResultSet::ptr results_ptr = select->executeQuery();
    dbc::ResultSet& results = *results_ptr;

    results.next();

    Test::assertEqual<int>(
            "Getting ints by label-based subscript operator works",
            results["age"], 42);