Cannot slice a SparseDataFrame's columns.
Example:

``` python
sdf = pandas.SparseDataFrame(index=[0, 1, 2], columns=['a', 'b','c'])

sdf[['a', 'b']]

/usr/lib64/python2.7/site-packages/pandas/sparse/frame.pyc in __getitem__(self, item)
    321         try:
    322             # unsure about how kludgy this is
--> 323             s = self._series[item]
    324             s.name = item
    325             return s

TypeError: unhashable type: 'list'

```
