ENH: (GH4629) Support for using a DatetimeIndex/PeriodsIndex directly in a datelike calculation

closes #4629

```
In [1]: s = Series(date_range('20130101',periods=5),index=date_range('20130102',periods=5))

In [2]: s
Out[2]: 
2013-01-02   2013-01-01 00:00:00
2013-01-03   2013-01-02 00:00:00
2013-01-04   2013-01-03 00:00:00
2013-01-05   2013-01-04 00:00:00
2013-01-06   2013-01-05 00:00:00
Freq: D, dtype: datetime64[ns]

In [3]: s-s.index
Out[3]: 
2013-01-02   -1 days, 00:00:00
2013-01-03   -1 days, 00:00:00
2013-01-04   -1 days, 00:00:00
2013-01-05   -1 days, 00:00:00
2013-01-06   -1 days, 00:00:00
Freq: D, dtype: timedelta64[ns]
```