ENH: function to take DatetimeIndex and adjust for leapseconds
[leapseconds list here](https://github.com/eggert/tz/blob/master/leap-seconds.list).

[SO discussion here](http://stackoverflow.com/questions/19332902/extract-historic-leap-seconds-from-tzdata). This info is actually available from the tzinfo, though I don't think `pytz` is reading it.

[astropy docs](http://docs.astropy.org/en/stable/api/astropy.time.TimeGPS.html)

This is just a matter of do a timedelta addition against this list (which could be hardcoded) and returning a new DTI. In theory this could be in the 'leapseconds' tz `right/UTC`, but that might require more work. Just thinking of a simple.

```
index = pd.date_range('.....')
index.adjusted_for_leapseconds()
```
