timedelta64[ns] : argmin() failing
argmax() works fine, but argmin() throws an error:

``` python
In [22]: diff = ts - ts.shift()

In [23]: diff
Out[23]:
0                NaT
1    00:00:00.128001
2    00:00:00.127999
3    00:00:00.128001
4    00:00:00.127999
5    00:00:00.128001
6    00:00:00.127999
7    00:00:00.128001
8    00:00:00.127999
9    00:00:00.128001
10   00:00:00.127999
11   00:00:00.128001
12   00:00:00.128000
13   00:00:00.127999
14   00:00:00.128001
...
25646   00:00:00.128001
25647   00:00:00.128000
25648   00:00:00.128000
25649   00:00:00.128000
25650   00:00:00.127999
25651   00:00:00.128000
25652   00:00:00.128000
25653   00:00:00.128000
25654   00:00:00.128001
25655   00:00:00.128000
25656   00:00:00.128000
25657   00:00:00.127999
25658   00:00:00.128000
25659   00:00:00.128000
25660   00:00:00.128000
Length: 25661, dtype: timedelta64[ns]

In [24]: diff.argmax()
Out[24]: 20540

In [25]: diff.argmin()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-25-e4a9e608a754> in <module>()
----> 1 diff.argmin()

/usr/local/epd/lib/python2.7/site-packages/pandas/core/series.pyc in wrapper(self, other)
    134
    135             else:
--> 136                 raise ValueError('cannot operate on a series with out a rhs '
    137                                  'of a series/ndarray of type datetime64[ns] '
    138                                  'or a timedelta')

ValueError: cannot operate on a series with out a rhs of a series/ndarray of type datetime64[ns] or a timedelta
In [26]: pd.__version__
Out[26]: '0.11.0.dev-819e0ad'
```
