Datetime series cannot subtract numpy.datetime64
For some reason, it's not possible to subtract a `numpy.datetime64` object from a `Series`, even though subtracting a `datetime.datetime` object is possible. I found this bug when trying to work around issue #8554.
#### Code Sample, a copy-pastable example if possible

``` python
import pandas as pd
import numpy as np
from datetime import datetime

EPOCH = datetime(1970, 1, 1)
EPOCH64 = np.datetime64(EPOCH)

df = pd.DataFrame(pd.date_range('2014-11-19', '2014-11-21', freq='D'))

epoch_ts = df[0] - EPOCH                                       # Works
epoch_tsnp = pd.Series(df[0].values - EPOCH64, name=0)         # Works
epoch_ts64 = df[0] - EPOCH64                                   # TypeError
```
#### Expected Output

`epoch_ts` and `epoch_tsnp` are both:

```
0   16393 days
1   16394 days
2   16395 days
Name: 0, dtype: timedelta64[ns]
```

There is a `TypeError` raised when `epoch_ts64` is calculated. A full stack trace follows:

```
~/anaconda3/lib/python3.5/site-packages/pandas/core/ops.py in wrapper(left, right, name, na_op)
    605 
    606         time_converted = _TimeOp.maybe_convert_for_time_op(left, right, name,
--> 607                                                            na_op)
    608 
    609         if time_converted is None:

~/anaconda3/lib/python3.5/site-packages/pandas/core/ops.py in maybe_convert_for_time_op(cls, left, right, name, na_op)
    565             return None
    566 
--> 567         return cls(left, right, name, na_op)
    568 
    569 

~/anaconda3/lib/python3.5/site-packages/pandas/core/ops.py in __init__(self, left, right, name, na_op)
    279 
    280         lvalues = self._convert_to_array(left, name=name)
--> 281         rvalues = self._convert_to_array(right, name=name, other=lvalues)
    282 
    283         self.name = name

~/anaconda3/lib/python3.5/site-packages/pandas/core/ops.py in _convert_to_array(self, values, name, other)
    400             if (supplied_dtype is None and other is not None and
    401                 (other.dtype in ('timedelta64[ns]', 'datetime64[ns]')) and
--> 402                     isnull(values).all()):
    403                 values = np.empty(values.shape, dtype='timedelta64[ns]')
    404                 values[:] = iNaT

~/anaconda3/lib/python3.5/site-packages/pandas/core/common.py in isnull(obj)
    174     pandas.notnull: boolean inverse of pandas.isnull
    175     """
--> 176     return _isnull(obj)
    177 
    178 

~/anaconda3/lib/python3.5/site-packages/pandas/core/common.py in _isnull_new(obj)
    184         raise NotImplementedError("isnull is not defined for MultiIndex")
    185     elif isinstance(obj, (ABCSeries, np.ndarray, pd.Index)):
--> 186         return _isnull_ndarraylike(obj)
    187     elif isinstance(obj, ABCGeneric):
    188         return obj._constructor(obj._data.isnull(func=isnull))

~/anaconda3/lib/python3.5/site-packages/pandas/core/common.py in _isnull_ndarraylike(obj)
    275         result = values.view('i8') == tslib.iNaT
    276     else:
--> 277         result = np.isnan(values)
    278 
    279     # box

TypeError: ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Linux
OS-release: 3.19.0-58-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.1
setuptools: 20.2.2
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.0.3
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.3
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.6
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.5.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.11
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
```
