API/ENH: relabel method
Alt to #15029

 - [x] closes #14829
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

small example
```python
In [7]: df = pd.DataFrame({'a': [1,2], 'b': [3, 4]})

In [8]: df.relabel(columns=['J', 'K'])
Out[8]: 
   J  K
0  1  3
1  2  4
```
 
