Can't select from MultiIndex using .xs()
``` python
df_param.index.levels[3]
>>> Index([u'a', u't', u't_anti', u'v_anti', u'v_pro', u'v_stop'], dtype='object')
df_param.xs(u'a', level=3)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-103-99e9d1eaffc4> in <module>()
----> 1 df_param.xs(u'a', level=3)

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/generic.pyc in xs(self, key, axis, level, copy, drop_level)
   1278             if axis == 1 and loc_is_slice:
   1279                 indexer = slice(None), indexer
-> 1280             result = self.ix[indexer]
   1281             setattr(result, result._get_axis_name(axis), new_ax)
   1282             return result

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/indexing.pyc in __getitem__(self, key)
     52                 pass
     53 
---> 54             return self._getitem_tuple(key)
     55         else:
     56             return self._getitem_axis(key, axis=0)

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/indexing.pyc in _getitem_tuple(self, tup)
    595     def _getitem_tuple(self, tup):
    596         try:
--> 597             return self._getitem_lowerdim(tup)
    598         except IndexingError:
    599             pass

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/indexing.pyc in _getitem_lowerdim(self, tup)
    690                     raise e1
    691                 try:
--> 692                     loc = ax0.get_loc(tup[0])
    693                 except KeyError:
    694                     raise e1

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/index.pyc in get_loc(self, key)
   3096                 return result
   3097         else:
-> 3098             return self._get_level_indexer(key, level=0)
   3099 
   3100     def get_loc_level(self, key, level=0, drop_level=True):

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/index.pyc in _get_level_indexer(self, key, level)
   3233     def _get_level_indexer(self, key, level=0):
   3234         level_index = self.levels[level]
-> 3235         loc = level_index.get_loc(key)
   3236         labels = self.labels[level]
   3237 

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/core/index.pyc in get_loc(self, key)
   1015         loc : int if unique index, possibly slice or mask if not
   1016         
-> 1017         return self._engine.get_loc(_values_from_object(key))
   1018 
   1019     def get_value(self, series, key):

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/index.so in pandas.index.IndexEngine.get_loc (pandas/index.c:3620)()

/home/ipython/anaconda/lib/python2.7/site-packages/pandas/index.so in pandas.index.IndexEngine.get_loc (pandas/index.c:3294)()

TypeError: 
```

Am I using it correctly or is this supposed to work like that?
