timedelta64[ns]: max() and min() return numpy.int64
I guess that max() and min() are not implementing yet fully:

``` python

In [28]: diff.head()
Out[28]:
0               NaT
1   00:00:00.128001
2   00:00:00.127999
3   00:00:00.128001
4   00:00:00.127999
Dtype: timedelta64[ns]

In [29]: diff.max()
Out[29]: 1934745001000

In [30]: diff.max().dtype
Out[30]: dtype('int64')

In [31]: diff[0].dtype
Out[31]: dtype('timedelta64[ns]')

In [32]: diff.min().dtype
Out[32]: dtype('int64')
```
