Bug: Selection of Multi-Index Columns
Hi everybody,

lets take the following DataFrame:

```
df = pd.DataFrame({
'Region' : ['A', 'A', 'C', 'C', 'C', 'B'],
'Store' : ['A', ' B ', 'A ', 'B ', 'A ', 'A'],
'Quantity': [1, 1, 2, 3, 4, 5],
'Date' : [
'2009-04-05', '2009-04-05', '2010-05-05', '2009-08-05',
'2011-02-05', '2009-04-05']})
df['Date'] = df['Date'].apply(lambda x: pd.to_datetime(x))
df.set_index(['Date','Region','Store'], inplace=True)
df_1 = df.unstack(level=[1,2])
```

Now when I try to query:

```
df_1['Quantity']['A']['A']
```

or

```
df_1['Quantity', 'A', 'A']
```

I get the correct subset, but if I query

```
df_1['Quantity']['A']['B']
```

pandas raises an exception.

What do you think about this?
