rolling( window='10D') does not work for df with MultiIndex

```python
# Your code here

tdf.rolling( '10D' ).mean()
```

```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-49-707b0ff61efc> in <module>()
     10 tdf = df.groupby(level=1).get_group( 64413 )
     11 
---> 12 tdf.rolling( '10D' ).mean()
     13 
     14 tdf.reset_index().set_index('time').rolling( '10D' ).mean()

/home/firdaus/.conda/envs/tsconda/lib/python3.5/site-packages/pandas/core/generic.py in rolling(self, window, min_periods, freq, center, win_type, on, axis)
   5502                                    min_periods=min_periods, freq=freq,
   5503                                    center=center, win_type=win_type,
-> 5504                                    on=on, axis=axis)
   5505 
   5506         cls.rolling = rolling

/home/firdaus/.conda/envs/tsconda/lib/python3.5/site-packages/pandas/core/window.py in rolling(obj, win_type, **kwds)
   1797         return Window(obj, win_type=win_type, **kwds)
   1798 
-> 1799     return Rolling(obj, **kwds)
   1800 
   1801 

/home/firdaus/.conda/envs/tsconda/lib/python3.5/site-packages/pandas/core/window.py in __init__(self, obj, window, min_periods, freq, center, win_type, axis, on, **kwargs)
     76         self.win_type = win_type
     77         self.axis = obj._get_axis_number(axis) if axis is not None else None
---> 78         self.validate()
     79 
     80     @property

/home/firdaus/.conda/envs/tsconda/lib/python3.5/site-packages/pandas/core/window.py in validate(self)
   1055 
   1056         elif not is_integer(self.window):
-> 1057             raise ValueError("window must be an integer")
   1058         elif self.window < 0:
   1059             raise ValueError("window must be non-negative")

ValueError: window must be an integer
```


#### Problem description

The offset feature of specifying timelike windows in 'rolling' doesn't work if the dataframe has multindex with level_0 = 'time' and level_1 = something else.


#### Expected Output
``` python
tdf.reset_index().set_index('time').rolling( '10D' ).mean()
```
Works correctly.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 3.16.0-0.bpo.4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2.post+ts3
nose: None
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.4
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: None
pandas_datareader: None
</details>
