DataFrame[sparse].__getitem__ should be Series, not SparseSeries
Can we break API here to return a Series[Sparse] instead?

```python
In [12]: df = pd.DataFrame({"A": pd.SparseArray([1, 2, 3])})

In [13]: df.dtypes
Out[13]:
A    Sparse[int64, 0]
dtype: object

In [14]: type(df['A'])
Out[14]: pandas.core.sparse.series.SparseSeries

```

cc @rkern 