global does not support getTimezoneOffset

[Sydney-o9]

Bug

There is no support for getTimezoneOffset

Debug

The following won't return the js function.

// Easy case with simple function
$func = new Expr('function () { return -10 * 60; }');
$ob = new Highchart();
$ob->global->getTimezoneOffset($func);

This is because renderGlobal() is not setup to be rendered by 'renderWithJavascriptCallback'

Use

This is very useful because if supported, we can do the following:

$func = new Expr("function (timestamp) {
        var zone = 'Australia/Sydney',
        timezoneOffset = -moment.tz(timestamp, zone).utcOffset();

        return timezoneOffset;
}");
$ob = new Highchart();
$ob->global->getTimezoneOffset($func);

Also, this would fix #92