BUG: coercion of non-M8[ns] in datetime ops
```
import datetime
s = pd.Series(pd.date_range('20130101',periods=3))
s-pd.Timestamp('20130101')
s-datetime.datetime(2013,1')
```

This fails as the datetime64 is not converted properly (because numpy datetime ops suck)

```
s-np.datetime64('20130101')
```

e.g. `np.datetime64('20130101').astype('M8[ns]')` is a bug, no?
