Fix handling of UTC and "floating" tz.

moment.toDate() is replaced with manually building the date from components, then setting the correct timezone.
If a "floating" timezone is found, it is replaced with the local timezone according it `ICAL.Timezone.localTimezone`.
If `start` or `end` are in the UTC timezone, do not set the tzid parameter on the `dtstart` (or `dtend`) property. The TZID is not needed because the timestamp string will actually end with a "Z" indicating UTC timezone.