Feature Sugesstion: date_range from end to start
When using `pd.date_range` we work our way forwards from start to end.  It would be nice to pass a flag or use the dates in reverse to go from end to start when creating the date_range.

``` python
#EX:
pd.date_range(end, start)
#or
pd.date_range(start, end, end_to_start=True)
```

``` python
Ex:
start = datetime.datetime(2016 ,2, 8)
end = datetime.datetime(2018 , 6, 1)
pd.date_range(start end, freq='6m', start_to_end=True)
>>> DatetimeIndex(['2016-02-08', '2016-06-01', '2016-12-01', '2017-06-01',
               '2017-12-01', '2018-06-01'])
```
