DataFrame.groupby.expanding duplicates grouped dimensions
While using a `groupby`, applying an `expanding` function duplicates the grouped dimensions.  I would expect the result of this to work like `groupby.cumsum` where the `expanding` function is applied over the groups and then the result has the same index as the original data frame.
#### Code Sample, a copy-pastable example if possible

My Data Frame sample

```
>>> dataframe
           one
cont cat1     
0    a       0
     b       1
1    a       2
     b       3
2    a       4
     b       5
3    a       6
     b       7
4    a       8
     b       9
5    a      10
     b      11
6    a      12
     b      13
7    a      14
     b      15
```

Example of the function call and incorrect output.

```
>>> dataframe.groupby(level=1).expanding(min_periods=1).mean()

                one
cat1 cont cat1     
a    0    a     0.0
     1    a     1.0
     2    a     2.0
     3    a     3.0
     4    a     4.0
     5    a     5.0
     6    a     6.0
     7    a     7.0
b    0    b     1.0
     1    b     2.0
     2    b     3.0
     3    b     4.0
     4    b     5.0
     5    b     6.0
     6    b     7.0
     7    b     8.0
```
#### Expected Output

```
>>> dataframe.groupby(level=1).cumsum()

           one
cont cat1     
0    a       0.0
     b       1.0
1    a       1.0
     b       2.0
2    a       2.0
     b       3.0
3    a      3.0
     b      4.0
4    a      4.0
     b      5.0
5    a      5.0
     b      6.0
6    a      6.0
     b      7.0
7    a      7.0
     b      8.0
```
#### output of `pd.show_versions()`

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.0.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

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 18.1
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.2
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
```
