MultiIndex partial slicing error
```
In [10]: idx = MultiIndex(labels=[[0,0,0],[0,1,1],[1,0,1]],levels=[['a','b'],['x','y'],['p','q']])

In [11]: df = DataFrame(np.random.rand(3,2),index=idx)

In [12]: df.ix[('a','y')]
Out[12]: 
      0         1
p  0.788191  0.781931
q  0.566332  0.486645

In [13]: df[('a','y')]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/home/adam/code/qplib/<ipython-input-13-d3c63a882cb1> in <module>()
----> 1 df[('a','y')]

/home/adam/code/pandas/pandas/core/frame.pyc in __getitem__(self, key)
   1655             return self._getitem_multilevel(key)
   1656         else:
-> 1657             return self._get_item_cache(key)
   1658 
   1659     def _getitem_array(self, key):

/home/adam/code/pandas/pandas/core/generic.pyc in _get_item_cache(self, item)
    472         except Exception:
    473             try:
--> 474                 values = self._data.get(item)
    475                 res = self._box_item_values(item, values)
    476                 cache[item] = res

/home/adam/code/pandas/pandas/core/internals.pyc in get(self, item)
    756 
    757     def get(self, item):
--> 758         _, block = self._find_block(item)
    759         return block.get(item)
    760 

/home/adam/code/pandas/pandas/core/internals.pyc in _find_block(self, item)
    846 
    847     def _find_block(self, item):
--> 848         self._check_have(item)
    849         for i, block in enumerate(self.blocks):
    850             if item in block:

/home/adam/code/pandas/pandas/core/internals.pyc in _check_have(self, item)
    853     def _check_have(self, item):
    854         if item not in self.items:
--> 855             raise KeyError('no item named %s' % str(item))
    856 
    857     def reindex_axis(self, new_axis, method=None, axis=0, copy=True):

KeyError: "no item named ('a', 'y')"

In [14]: df.ix[('a','y'),:]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/home/adam/code/qplib/<ipython-input-14-19a9d60d6a3b> in <module>()
----> 1 df.ix[('a','y'),:]

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

/home/adam/code/pandas/pandas/core/indexing.pyc in _getitem_lowerdim(self, tup)
    165         if isinstance(ax0, MultiIndex):
    166             try:
--> 167                 return self._get_label(tup, axis=0)
    168             except TypeError:
    169                 # slices are unhashable


/home/adam/code/pandas/pandas/core/indexing.pyc in _get_label(self, label, axis)
     39             return self.obj.xs(label, axis=axis, copy=False)
     40         except Exception:
---> 41             return self.obj.xs(label, axis=axis, copy=True)
     42 
     43     def _slice(self, obj, axis=0):

/home/adam/code/pandas/pandas/core/frame.pyc in xs(self, key, axis, level, copy)
   1894         index = self.index
   1895         if isinstance(index, MultiIndex):
-> 1896             loc, new_index = self.index.get_loc_level(key)
   1897         else:
   1898             loc = self.index.get_loc(key)

/home/adam/code/pandas/pandas/core/index.pyc in get_loc_level(self, key, level)
   2000                 for i, k in enumerate(key):
   2001                     if not isinstance(k, slice):
-> 2002                         k = self._get_level_indexer(k, level=i)
   2003                         if isinstance(k, slice):
   2004                             # everything


/home/adam/code/pandas/pandas/core/index.pyc in _get_level_indexer(self, key, level)
   2030     def _get_level_indexer(self, key, level=0):
   2031         level_index = self.levels[level]
-> 2032         loc = level_index.get_loc(key)
   2033         labels = self.labels[level]
   2034 

/home/adam/code/pandas/pandas/core/index.pyc in get_loc(self, key)
    623         loc : int
    624         """
--> 625         return self._engine.get_loc(key)
    626 
    627     def get_value(self, series, key):

/home/adam/code/pandas/pandas/lib.so in pandas.lib.IndexEngine.get_loc (pandas/src/tseries.c:112968)()

/home/adam/code/pandas/pandas/lib.so in pandas.lib.IndexEngine.get_loc (pandas/src/tseries.c:112835)()

/home/adam/code/pandas/pandas/lib.so in pandas.lib.PyObjectHashTable.get_item (pandas/src/tseries.c:22698)()

/home/adam/code/pandas/pandas/lib.so in pandas.lib.PyObjectHashTable.get_item (pandas/src/tseries.c:22652)()

KeyError: ('a', 'y')
```
