Cannot set a datetime64 column to datetime.max
#### Code Sample, a copy-pastable example if possible

``` python
import datetime
import pandas as pd

df = pd.DataFrame()
df['string_date'] = ['2015-01-01', '']
df['timestamp'] = df.string_date.values.astype("datetime64[M]")
df.loc[df.timestamp.isnull(), 'timestamp'] = datetime.datetime.max
```

Error

```
OutOfBoundsDatetime: Out of bounds nanosecond timestamp: 9999-12-31 23:59:59
```
#### Expected Output

No error
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 4.5.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.2
setuptools: 20.10.1
Cython: 0.24
numpy: 1.11.0
scipy: 0.17.1
statsmodels: None
xarray: None
IPython: 4.0.3
sphinx: 1.3.6
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: 0.7.3
lxml: 3.6.0
bs4: 4.4.1
html5lib: 0.999
httplib2: 0.9.1
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
