Unhelpful exception / bug in ix + MultiIndex use case
This should work IMHO

```
In [20]: df
Out[20]: 
       value
   id       
t1 a   1    
   b   2    
   c   3    
t2 a   7    
   b   8    

In [21]: df.ix[:, 'value']
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/<ipython-input-21-7d089958dc90> in <module>()
----> 1 df.ix[:, 'value']

/home/wesm/code/pandas/pandas/core/indexing.pyc in __getitem__(self, key)
     28                 pass
     29 
---> 30             return self._getitem_tuple(key)
     31         else:
     32             return self._getitem_axis(key, axis=0)

/home/wesm/code/pandas/pandas/core/indexing.pyc in _getitem_tuple(self, tup)
     97     def _getitem_tuple(self, tup):
     98         try:
---> 99             return self._getitem_lowerdim(tup)
    100         except IndexingError:
    101             pass

/home/wesm/code/pandas/pandas/core/indexing.pyc in _getitem_lowerdim(self, tup)
    126                 pass
    127             except Exception:
--> 128                 if tup[0] not in ax0:
    129                     raise
    130 

/home/wesm/code/pandas/pandas/core/index.pyc in __contains__(self, key)
    209 
    210     def __contains__(self, key):
--> 211         return key in self._engine
    212 
    213     def __hash__(self):

/home/wesm/code/pandas/pandas/_engines.so in pandas._engines.DictIndexEngine.__contains__ (pandas/src/engines.c:1958)()

TypeError: unhashable type
```
