NaT in MultiIndex throws ValueError when reset_index() is called.
Please see this test case:

```
x = pp.DataFrame({
    'other': ['a', np.nan],
    'date': ['2015-03-22', np.nan],
    'amount': [2, 3]
  })
x.date = pp.to_datetime(x.date)
```

| index | amount | date | other |
| --- | --- | --- | --- |
| 0 | 2 | 2015-03-22 | a |
| 1 | 3 | NaT | NaN |

The reset_index here then fails with the error below:

```
x.set_index(['other', 'date']).reset_index()

ValueError: Converting an integer to a NumPy datetime requires a specified unit
```

pp.version.version
'0.16.1'
pp.np.version.version
'1.7.1'
