Concat does not work for sparse series
Concatenating two sparse series does not return sparse data structures as expected:

```
In [1]: import pandas as pd

In [2]: ts = pd.Series([0, 1, 1, 2, 3, 0 ,0 ,0])

In [3]: sts = ts.to_sparse()

In [4]: print type(pd.concat([sts, sts], axis=0))
<class 'pandas.core.series.Series'>

In [5]: print type(pd.concat([sts, sts], axis=1))
<class 'pandas.core.frame.DataFrame'>
```

The above _does_ work correctly for SparseDataFrames.
