FIX: Scalar timedelta NaT results raise
Currently, coercion of scalar results from `float` to `timedelta64[ns]` passes through `int`, which raises when attempting to coerce `NaN` to `NaT`.

To reproduce:

``` python

import pandas as pd
import numpy as np
pd.Series([np.timedelta64('NaT')]).sum()
# TypeError: reduction operation 'sum' not allowed for this dtype
```
