DOC/TST: Series.reorder_levels() can take names; added tests

Minor change to the docstring. It said you must refer to the position (not keys). Keys work:

```python
In [18]: res.head()
Out[18]: 
same_employer  stamp     
-3             1996-02-01        2
-2             1996-01-01        1
               1996-02-01        1
               1996-03-01        4
 1             1996-01-01    13877
dtype: int64

In [19]: res.reorder_levels(['stamp', 'same_employer']).head()
Out[19]: 
stamp       same_employer
1996-02-01  -3                   2
1996-01-01  -2                   1
1996-02-01  -2                   1
1996-03-01  -2                   4
1996-01-01   1               13877
dtype: int64
```