datetime64/timedelta64 do not accept numpy integer types

Example:
```python
>>> np.datetime64(1478476800, "s")
numpy.datetime64('2016-11-07T00:00:00')

>>> np.datetime64(np.int64(1478476800), "s")
ValueError: Could not convert object to NumPy datetime

>>> np.timedelta64(1, "s")
numpy.timedelta64(1,'s')

>>> np.timedelta64(np.int64(1), "s")
ValueError: Could not convert object to NumPy timedelta
```