Inconsistent slicing with hierarchical indexing
Running the following code:

``` python
import pandas as pd
frame = pd.DataFrame(np.arange(6*6).reshape((6,6)),\
                     index=[['a', 'a', 'b', 'b', 'c', 'c'], [1, 2, 1, 2, 1, 2]],\
                     columns=[['a', 'a', 'b', 'b', 'c', 'c'], [1, 2, 1, 2,1,2]])
print frame['a']
print frame['a':'b']
print frame[['a','b']]
```

gives as output:

```
   1   2
```

a 1   0   1
   2   6   7
b 1  12  13
   2  18  19
c 1  24  25
   2  30  31
       a        b         c  
       1   2    1    2   1   2
a 1   0   1    2   3   4   5
   2   6   7    8   9  10  11
b 1  12  13  14 15  16  17
   2  18  19  20 21  22  23
      a       b  
      1   2   1   2
a 1   0   1   2   3
  2   6   7   8   9
b 1  12  13  14  15
  2  18  19  20  21
c 1  24  25  26  27
  2  30  31  32  33

If I understand correctly how this should work. frame[...] should be selecting from the columns. However, as can be seen, the result of the second print statement selects rows from the dataframe.
Thus print frame['a':'b'] does not give the correct output.
