pd.expanding is incorrectly calculating window size when axis=1
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
import numpy as np

df = pd.DataFrame({'A': [0, 1, 2, np.nan, 4], 
                   'B': [0, 1, 2, np.nan, 4], 
                   'C': [0, 1, 2, np.nan, 4], 
                   'D': [0, 1, 2, np.nan, 4], 
                   'E': [0, 1, 2, np.nan, 4], 
                   'F': [0, 1, 2, np.nan, 4]})

print df.expanding(axis=1).sum()
```
#### Expected Output

```
     A    B     C     D     E     F
0  0.0  0.0   0.0   0.0   0.0   0.0
1  1.0  2.0   3.0   4.0   5.0   5.0
2  2.0  4.0   6.0   8.0  10.0  10.0
3  NaN  NaN   NaN   NaN   NaN   NaN
4  4.0  8.0  12.0  16.0  20.0  20.0
```

However, the correct result should be:

```
     A    B     C     D     E     F
0  0.0  0.0   0.0   0.0   0.0   0.0
1  1.0  2.0   3.0   4.0   5.0   6.0
2  2.0  4.0   6.0   8.0  10.0  12.0
3  NaN  NaN   NaN   NaN   NaN   NaN
4  4.0  8.0  12.0  16.0  20.0  24.0
```

Notice that the last column `E` is different. I've tracked this down and found that the _get_window function (for expanding) fails to return the correct number of windows when the following conditions are met:
1. `axis=1` is used instead of `axis=0` (default)
2. The number of rows in the dataframe is less than the number of columns

This is caused by the fact that the object is using `len(obj)` in determining the window size. Instead, it should be using `obj.shape[self.axis]` 
#### output of `pd.show_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: None
LOCALE: None.None

pandas: 0.18.1+237.ge357ea1
nose: 1.3.7
pip: 8.1.2
setuptools: 20.1.1
Cython: 0.23.4
numpy: 1.11.1
scipy: 0.17.1
statsmodels: 0.6.1
xarray: 0.7.0
IPython: 4.0.3
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.4.1
pytz: 2015.7
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.6.0
matplotlib: None
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.5.0
bs4: 4.4.1
html5lib: None
httplib2: 0.9
apiclient: 1.4.0
sqlalchemy: 1.0.11
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext)
jinja2: 2.8
boto: 2.39.0
pandas_datareader: 0.2.0
