argmin/argmax, idxmin/idxmax and datetime64
``` python
In [65]: s
Out[65]: 
a   2013-01-02 00:00:00
b   2013-01-03 00:00:00
c   2013-01-04 00:00:00
d   2013-01-05 00:00:00
e                   NaT
dtype: datetime64[ns]

In [66]: s.argmin()
Out[66]: 4

In [67]: s.idxmin()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
...
TypeError: ufunc 'isfinite' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule 'safe'

In [68]: s.argmax()
Out[68]: 3

In [69]: s.idxmax()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
...
TypeError: ufunc 'isfinite' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule 'safe'

```
