Add option to specified the index's name in pd.Series.rename.
Suppose I have a series:

``` python
In [31]: pd.__version__
Out[31]: u'0.17.1'

In [32]: ser = pd.Series(range(4), index=['Aa', 'Ab', 'Ba', 'Bb'])

In [33]: ser
Out[33]: 
Aa    0
Ab    1
Ba    2
Bb    3
dtype: int64
```

I can change the index as following:

``` python
In [34]: ser.rename(lambda i: tuple(i))
Out[34]: 
A  a    0
   b    1
B  a    2
   b    3
dtype: int64

In [35]: ser.index.name = 'First', 'Second'
In [36]: ser.rename(lambda i: tuple(i))    
Out[36]: 
First  Second
A      a         0
       b         1
B      a         2
       b         3
dtype: int64
```

But `rename` failed if the index's name is a single str:

``` python
In [37]: ser.index.name = 'Title'
In [38]: ser.rename(lambda i: tuple(i))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-38-72ab91ddd5a1> in <module>()
----> 1 ser.rename(lambda i: tuple(i))
-> 4371                 'Length of names must match number of levels in MultiIndex.')
   4372 
   4373         if level is None:
ValueError: Length of names must match number of levels in MultiIndex.
In [39]: 
```

It will be good if the `rename` method has option like `indexNames` and can be used like

``` python
In [39]: ser.rename(lambda i: tuple(i), indexNames=['First', 'Second'])
```
