Series with dtype='M8[us]' does not create NaT values
Using Pandas 0.10.1, python 2.7, and Numpy 1.6.2:

```
>>> npa = numpy.ndarray(1, dtype=numpy.int64)
>>> npa[0] = -9223372036854775808
>>> type(pandas.Series(npa, dtype='M8[ns]')[0])
<class 'pandas.tslib.NaTType'>
>>> type(pandas.Series(npa, dtype='M8[us]')[0])
<class 'pandas.tslib.Timestamp'>
```

The latter should also have class 'pandas.tslib.NaTType', right?
