BUG: GH3109 fixed issues where passing an axis of 'columns' would fail
BUG: some operations were expecting an axis number, this fix allows passing the regular axis name as well, e.g. 
`df.sum(axis='columns')` will now work

_note: narrowed the scope of this PR to just a bug fix_

```
In [3]: df = pd.DataFrame(np.random.rand(5,2),columns=['A','B'])

In [4]: df.sum(axis=1)
Out[4]: 
0    1.150325
1    0.789142
2    0.581486
3    0.864212
4    0.731511

In [5]: df.sum(axis='columns')
---------------------------------------------------------------------------

Exception: Must have 0<= axis <= 1

```
