hierarchical slices
Consider the following data:

``` python
In [36]: index = pd.MultiIndex.from_arrays([['a','a','b','b','c','c'], [1,2,1,2,1,2]], names=['first','second'])

In [37]: data = pd.DataFrame(np.random.rand(len(index),2), index=index, columns=['A','B'])

In [38]: data
Out[38]: 
                     A         B
first second                    
a     1       0.320885  0.794688
      2       0.062344  0.808103
b     1       0.789837  0.956867
      2       0.129658  0.461514
c     1       0.237009  0.975737
      2       0.779002  0.692922
```

Now, I can do slicing on the first index as:

``` python
In [40]: data.ix[['a','b']]
Out[40]: 
                     A         B
first second                    
a     1       0.320885  0.794688
      2       0.062344  0.808103
b     1       0.789837  0.956867
      2       0.129658  0.461514
```

I can also select a particular first, and second index as

``` python
In [41]: data.ix['a',1]
Out[41]: 
A    0.320885
B    0.794688
Name: (a, 1), dtype: float64
```

However, if I try to take an arbitrary slice through the first and second index, it doesn't work, but something unexpected, it selects a column!

``` python
In [42]: data.ix[['a','b'],1]
Out[42]: 
first  second
a      1         0.794688
       2         0.808103
b      1         0.956867
       2         0.461514
Name: B, dtype: float64
```

when I was expecting

``` python
                 A         B
first                    
a         0.320885  0.794688
b         0.789837  0.956867
```
