Make NatType more Timestamp-like and less datetime.datetime-like
``` python
In [127]: isinstance(pd.Timestamp(np.nan), pd.tslib.Timestamp)
Out[127]: False

In [128]: isinstance(pd.Timestamp('2015'), pd.tslib.Timestamp)
Out[128]: True

In [129]: isinstance(pd.Timestamp(np.nan), datetime.datetime)
Out[129]: True

In [130]: isinstance(pd.Timestamp('2015'), datetime.datetime)
Out[130]: True
```
