BUG: Multiple timedetal64[ns] operations causes changes in dtypes
ok

```
In [34]: df = pd.DataFrame({ 'time' : pd.date_range('20130102',periods=5), 
                                         'time2' : pd.date_range('20130105',periods=5) })

In [35]: df['off1'] = df['time2']-df['time']

In [36]: df.dtypes
Out[36]: 
time      datetime64[ns]
time2     datetime64[ns]
off1     timedelta64[ns]
dtype: object
```

dtypes of both existing are changed to `us`

```
In [37]: df['off2'] = df['time']-df['time2']

In [38]: df.dtypes
Out[38]: 
time      datetime64[ns]
time2     datetime64[ns]
off1     timedelta64[us]
off2     timedelta64[us]
dtype: object
```
