BUG: inconsistent behavior of set_value
when using set_value to update a DataFrame, the update is done inplace if the given label exist in the index, but a new DataFrame is created if the label is not in the index.

For instance:
The following will not update the original DataFrame

```
In [99]: a = pd.DataFrame(rand(5,5), columns=['a','b','c','d','e'])

In [100]: a
Out[100]:
          a         b         c         d         e
0  0.195670  0.357315  0.185997  0.555534  0.511327
1  0.638141  0.091384  0.970876  0.500370  0.522014
2  0.818335  0.779157  0.925812  0.443544  0.630370
3  0.188840  0.419840  0.088478  0.782161  0.059741
4  0.938865  0.884169  0.514422  0.279719  0.282072

In [101]: a.set_value(8,col='a',value=3)
Out[101]:
          a         b         c         d         e
0  0.195670  0.357315  0.185997  0.555534  0.511327
1  0.638141  0.091384  0.970876  0.500370  0.522014
2  0.818335  0.779157  0.925812  0.443544  0.630370
3  0.188840  0.419840  0.088478  0.782161  0.059741
4  0.938865  0.884169  0.514422  0.279719  0.282072
8  3.000000       NaN       NaN       NaN       NaN

In [102]: a
Out[102]:
          a         b         c         d         e
0  0.195670  0.357315  0.185997  0.555534  0.511327
1  0.638141  0.091384  0.970876  0.500370  0.522014
2  0.818335  0.779157  0.925812  0.443544  0.630370
3  0.188840  0.419840  0.088478  0.782161  0.059741
4  0.938865  0.884169  0.514422  0.279719  0.282072
```

However when using an existing label the update is done inplace:

```
In [103]: a.set_value(3,col='a',value=3)
Out[103]:
          a         b         c         d         e
0  0.195670  0.357315  0.185997  0.555534  0.511327
1  0.638141  0.091384  0.970876  0.500370  0.522014
2  0.818335  0.779157  0.925812  0.443544  0.630370
3  3.000000  0.419840  0.088478  0.782161  0.059741
4  0.938865  0.884169  0.514422  0.279719  0.282072

In [104]: a
Out[104]:
          a         b         c         d         e
0  0.195670  0.357315  0.185997  0.555534  0.511327
1  0.638141  0.091384  0.970876  0.500370  0.522014
2  0.818335  0.779157  0.925812  0.443544  0.630370
3  3.000000  0.419840  0.088478  0.782161  0.059741
4  0.938865  0.884169  0.514422  0.279719  0.282072
```
