pandas.DataFrame.rolling: can window be an int or offset, as suggested in the docs?
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
pd.__version__ # 0.20.1
# df is a DataFrame with datetime index
df.rolling(window='3D', freq='2D')
# returns the following error, inconsistent with latest docs:

  File "/Users/guest/anaconda/lib/python2.7/site-packages/pandas/core/window.py", line 1099, in validate
    raise ValueError("window must be an integer")



```
#### Problem description

The pandas 0.20.1 documentation for the rolling() method here:
https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.rolling.html
suggest that window may be an offset:
"window : int, or offset"
However, the code under core/window.py seems to suggest that window must be an int.

#### Expected Output
For basic examples of trying to sample a time series in Pandas 0.20.1 every 2 days with 3 day lookbacks,  see:
https://stackoverflow.com/questions/44234301/apply-rolling-function-to-groupby-over-several-columns

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.20.1
pytest: 2.6.4
pip: 9.0.1
setuptools: 35.0.2
Cython: 0.24
numpy: 1.12.1
scipy: 0.18.1
xarray: None
IPython: 5.3.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.6.2
feather: None
matplotlib: 1.5.1
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: None
html5lib: 0.999999999
sqlalchemy: 0.9.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
