[0.19dev] Time-series aware rolling window with MultiIndex with a time-frequency index fails
#### Code Sample, a copy-pastable example if possible

```
dft = pd.DataFrame({'B': [0, 1, 2, np.nan, 4]},
  index=pd.date_range('20130101 09:00:00', periods=5, freq='s'))
dft['another'] = ['a', 'a', 'c', 'd', 'e']
dft.set_index([dft.index, 'another'])
dft.rolling('2s')
```
#### Expected Output

Expected to be able to use a time-series aware rolling window, instead got `ValueError: window must be an integer`
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0rc1
nose: None
pip: 6.0.8
setuptools: 27.2.0
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
```
