Ensure rrule occurences have correct tzinfo

Datetime math does not adjust tzinfo of the result,  so, with:

vilnius = pytz.timezone('Europe/Vilnius')
dt = vilnius.localize(datetime.datetime(2014, 3, 30, 0, 0, 0))
result = dt + datetime.timedelta(days=2)

result will be:

datetime.datetime(2014, 4, 1, 0, 0, tzinfo=<DstTzInfo 'Europe/Vilnius' EET+2:00:00 STD>)

while it has to be:

datetime.datetime(2014, 4, 1, 0, 0, tzinfo=<DstTzInfo 'Europe/Vilnius' EEST+3:00:00 DST>)

While his approach _may_ be not absolutely correct in general, but in
this case (rrule) tzinfo should be adjusted while the time should
stay the same.