Add column name option to Series.reset_index
Currently the column name in the result DataFrame will either be 0 or the `name` attribute (if any):

```
In [31]: df1.groupby( [ "Name", "City"] ).size()
Out[31]: 
Name     City    
Alice    Seattle     1
Bob      Seattle     2
Mallory  Portland    2
         Seattle     1

In [32]: df1.groupby( [ "Name", "City"] ).size().reset_index()
Out[32]: 
      Name      City  0
0    Alice   Seattle  1
1      Bob   Seattle  2
2  Mallory  Portland  2
3  Mallory   Seattle  1
```
