DataFrame .sum() division now expands columns to include index
```
dic = {'01/31/2015': {'A': 5, 'B': 10, 'C': 15}, 
          '02/28/2015': {'A': 1, 'B': 20, 'C': 2}}
df = pd.DataFrame.from_dict(dic, orient='index')
df * 100 / df.sum(axis=1)

Out[33]:
            01/31/2015  02/28/2015   A   B   C
01/31/2015         NaN         NaN NaN NaN NaN
02/28/2015         NaN         NaN NaN NaN NaN
```

What used to occur here was this, which I assume is expected behavior:

```
df.apply(lambda x: x * 100 / x.sum(), axis=1)

Out[34]:
                                A          C          B
01/31/2015  16.666667  50.000000  33.333333
02/28/2015   4.347826   8.695652  86.956522
```
