renaudboyer

`$timezone->add('DAYLIGHT')` won't work as DAYLIGHT is not declared as a component of `VCalendar` because `$timezone->add()` have to detect type and eventually use its $root which is a VCalendar to create...

PERIOD deal with dates by itself when serializing to jCal ( [here](https://github.com/sabre-io/vobject/blob/122cacbdea2c6133ac04db86ec05854beef75adf/lib/Property/ICalendar/Period.php#L101) ). There are no TZ in PERIOD, they are handled by parent component like this: `RDATE;TZID=US/Eastern;VALUE=PERIOD:20060102T150000/PT2H`. PERIOD's dates...