Add time zone support, and %z and %Z arguments to the $dateToString operator

The $dateToString operator is extended with the timezone argument. Its full new syntax is now:

    { $dateToString: {

        format: formatString,

        date: dateExpression,

        timezone: timeZoneIdentifier

    } }

The timezone argument is optional. The "timezone" argument is optional, and must evaluate to specify an Olson Timezone Identifier such as "Europe/London" or "America/New_York", or, a UTC offset string in the forms: "+03", "-0530", and "+04:45". If present, it formats the string according to the given time zone, otherwise it uses UTC.

The $dateToString format arguments have also been expanded. With the addition of the timezone argument came the %z and %Z format specifiers:

%z
The +hhmm or -hhmm numeric timezone as a string (that is, the hour and minute offset from UTC). Example: ``+0445``, ``-0500``

%Z
The minutes offset from UTC as a number. Example (following the ``+0445`` and ``-0500`` from %z): ``+285``, ``-300``