tz_localize should support is_dst input array
When storing datetimes with timezone information in mysql I split out the is_dst flag into a separate column.  Then when reconstructing the Timestamps I am either forced to iterate through each row and call pytz.timezone.localize on every Timestamp which is very slow or do some magic with localizing what I can and then manually dealing with the fall transition time (note that infer_dst won't work because there could be many rows that have transitions in them).  I would much rather create the DatetimeIndex from the column of dates and then call tz_localize with the is_dst column.  This would then appropriately set the offset.

```
di = DatetimeIndex(frame['DateColumn'])
di = di.tz_localize(TimeZone, is_dst_flat=frame['IsDstColumn'])
```

Thoughts?
