DataFrame.set_value() issue
Accedentically swapping the index and column label in using `DataFrame.set_value()` gave an unexpected result.
All values, except the one to be set, in the _DataFrame_ became _NaN_.

``` python
In [4]: df
Out[4]:
          A         B         C
0  0.463981  0.163885  0.644793
1  0.979047  0.347373  0.736267
2  0.103155  0.304439  0.911314

In [5]: df.set_value('C', 2, 1.0)
Out[5]:
    A   B   C   2
0 NaN NaN NaN NaN
1 NaN NaN NaN NaN
2 NaN NaN NaN NaN
C NaN NaN NaN   1

In [6]: pandas.__version__
Out[6]: '0.7.2.dev-79decd7'
```
