DataFrame sort not stable
Passing a list of strings to the DataFrame.sort() method is stable, but passing a string by itself causes the sort to be unstable.  

I'm defining sort stability to mean row order is maintained for duplicate values in the sorted column.
## Code to recreate

```
>>>import pandas as pd
>>>df = pd.DataFrame(list('ABCDABCDABCD'), columns=['x'])
>>>print df.sort(['x'])
    x
0   A
4   A
8   A
1   B
5   B
9   B
2   C
6   C
10  C
3   D
7   D
11  D
>>>print df.sort('x')
    x
0   A
4   A
8   A
1   B
5   B
9   B
6   C
10  C
2   C
3   D
11  D
7   D
```

The index is out of order for C and D in the 2nd case.

pandas version 0.10.0
Python version 2.7.2
Windows 7, 64-bit
