This class is the central point to fetch timezone information from the
timezone (Olson) database, parse it, cache it, and generate VTIMEZONE
objects.
Usage:
$tz = new Horde_Timezone();
$tz->getZone('America/New_York')->toVtimezone()->exportVcalendar();
Documentation on the database file formats can be found at
ftp://ftp.iana.org/tz/tz-how-to.html