BUG: broadcasting subtraction
This bug applies to version `0.11.0`. I observe correct behavior in version `0.12.0.dev-84fb0e0`, so the issue seems to have been fixed. Still, I thought someone might find it useful to see this report.

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EDIT: Replaced a larger and more complicated example with a minimal example.

```
In [84]: df = pd.DataFrame({'A':['a1','a2','a3'],'B':[4,5,6]})

In [85]: df
Out[85]: 
    A  B
0  a1  4
1  a2  5
2  a3  6

In [86]: df.ix[[0,1], 'B'] -= 10

In [87]: df
Out[87]: 
    A  B
0  a1 -6
1  a2 -6
2  a3  6
```
