ical export breaks

[gyst]

I guess this is because 'Africa/Abidjan' is perhaps the first timezone?

  Module plone.app.event.ical.exporter, line 408, in __call__
  Module plone.app.event.ical.exporter, line 404, in get_ical_string
  Module zope.component.hooks, line 144, in adapter_hook
  Module plone.app.event.ical.exporter, line 177, in calendar_from_container
  Module plone.app.event.ical.exporter, line 65, in construct_icalendar
  Module plone.app.event.ical.exporter, line 144, in add_to_zones_map
  Module plone.app.event.ical.exporter, line 140, in localize
  Module pytz.tzinfo, line 204, in fromutc
OverflowError: date value out of range

(Pdb) tz
<DstTzInfo 'Africa/Abidjan' LMT-1 day, 23:44:00 STD>
(Pdb) prev_transition
datetime.datetime(1, 1, 1, 0, 0)
(Pdb) tzmap
{}
(Pdb) tzid
'Africa/Abidjan'
(Pdb) dt
datetime.datetime(2015, 2, 5, 0, 0, tzinfo=<DstTzInfo 'Africa/Abidjan' GMT0:00:00 STD>)

(Pdb) datetime.min
datetime.datetime(1, 1, 1, 0, 0)
(Pdb) prev_transition
datetime.datetime(1, 1, 1, 0, 0)
(Pdb) prev_transition is datetime.min
True
(Pdb) prev_transition is null
False

Fix forthcoming