Add method to change the .name attribute on the axes
It would be nice to have a method to change the `.name` attribute on the columns/index, to permit it in a method chain. (This is a suggestion based on [this stackoverflow question](http://stackoverflow.com/questions/34598733/dataframe-method-for-renaming-axis).)

Instead of 

```
df.columns.name = 'A'
df.index.name = 'B'
df
    A   0   1
    B        
    0   9   4
    1  15   0
    2  17  16
```

it would allow something like

```
df.rename_columns('A').rename_index('B')
```

or perhaps be a keyword argument to the existing `.rename()`. Does this seem like something that would fit in with pandas?
