np.sort(series) returns ndarray
Applying numpy sort function to a Series returns an ndarray when you would expect a Series (and got a Series in pandas 0.11.0). Tested in pandas 0.13.1.

``` python
s = pd.Series(np.arange(5.)[::-1])
np.sort(s)
Out[68]: array([ 0.,  1.,  2.,  3.,  4.])
```

This doesn't happen for all numpy functions e.g. 

``` python
np.power(s, 2)
Out[70]: 
0    16
1     9
2     4
3     1
4     0
dtype: float64
type(_)
Out[71]: pandas.core.series.Series
```
