Label slices on Series with MultiIndex
I have a Series with a MultiIndex like this:

```
In [174]: a
Out[174]: 
hour  year  weekday  workingday       
0     2011  0        0           count      4.000000
                                 mean      76.750000
                                 std       53.368998
                                 min       17.000000
                                 25%       44.000000
                                 50%       75.000000
                                 75%      107.750000
                                 max      140.000000
                     1           count     28.000000
                                 mean      29.500000
                                 std       31.792732
                                 min        4.000000
                                 25%       15.000000
                                 50%       24.500000
                                 75%       35.500000
...
23    2012  5        0           mean     152.687500
                                 std       55.711490
                                 min       20.000000
                                 25%      119.000000
                                 50%      148.000000
                                 75%      187.750000
                                 max      239.000000
            6        0           count     34.000000
                                 mean      71.617647
                                 std       32.471913
                                 min       23.000000
                                 25%       46.000000
                                 50%       68.500000
                                 75%      104.250000
                                 max      123.000000
Length: 3456, dtype: float64
```

And I'd like to get, for example, the rows where `hour=1, year=2011, weekday=[1,2,3,4]`

Intuitively, I would do this: `a.loc[1,2011,1:4]` which would work in a 4-dimensional numpy array. Unfortunately it raises the error:

``` pytb
/usr/lib/python2.7/dist-packages/pandas/core/index.pyc in get_loc_level(self, key, level, drop_level)
   3214                             continue
   3215                         else:
-> 3216                             raise TypeError(key)
   3217 
   3218                     if indexer is None:

TypeError: (0, 2011, slice(0, 4, None))
```

Even If I try suppling the index list instead of a slice: `a.loc[0,2011,[1,2,3,4]]` I get an error: `unhashable type: 'list'`
Is there a reason why this syntax is not supported? How can I select labels slices in a MultiIndex?
