Selecting columns via df[:, column_list] broken
happened with latest pandas.

```
from pandas import *
x = DataFrame([(0,1),(2,3)], columns = ['a','b'])
x[:,['a']]

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-2277db18002c> in <module>()
      1 from pandas import *
      2 x = DataFrame([(0,1),(2,3)], columns = ['a','b'])
----> 3 x[:,['a']]

/usr/local/lib/python2.7/dist-packages/pandas-0.8.0.dev_ed7a3d2-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in __getitem__(self, key)
   1679                 raise ValueError('Cannot index using non-boolean DataFrame')
   1680         else:
-> 1681             return self._get_item_cache(key)
   1682 
   1683     def _getitem_array(self, key):

/usr/local/lib/python2.7/dist-packages/pandas-0.8.0.dev_ed7a3d2-py2.7-linux-x86_64.egg/pandas/core/generic.pyc in _get_item_cache(self, item)
    471             return cache[item]
    472         except Exception:
--> 473             values = self._data.get(item)
    474             res = self._box_item_values(item, values)
    475             cache[item] = res

/usr/local/lib/python2.7/dist-packages/pandas-0.8.0.dev_ed7a3d2-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in get(self, item)
    783 
    784     def get(self, item):
--> 785         _, block = self._find_block(item)
    786         return block.get(item)
    787 

/usr/local/lib/python2.7/dist-packages/pandas-0.8.0.dev_ed7a3d2-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in _find_block(self, item)
    892 
    893     def _find_block(self, item):
--> 894         self._check_have(item)
    895         for i, block in enumerate(self.blocks):
    896             if item in block:

/usr/local/lib/python2.7/dist-packages/pandas-0.8.0.dev_ed7a3d2-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in _check_have(self, item)
    898 
    899     def _check_have(self, item):
--> 900         if item not in self.items:
    901             raise KeyError('no item named %s' % str(item))
    902 

/usr/local/lib/python2.7/dist-packages/pandas-0.8.0.dev_ed7a3d2-py2.7-linux-x86_64.egg/pandas/core/index.pyc in __contains__(self, key)
    276 
    277     def __contains__(self, key):
--> 278         hash(key)
    279         # work around some kind of odd cython bug
    280         try:

TypeError: unhashable type
```
