DEPR: deprecate cols / change to columns in to_csv / to_excel
Fix #6645

I think there are also some occurences in the docs:

```
>>>-------------------------------------------------------------------------
Warning in /tmp/doc/source/comparison_with_r.rst at block ending on line 175
Specify :okwarning: as an option in the ipython:: block to suppress this message
----------------------------------------------------------------------------
/home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/pandas/tools/pivot.py:82: FutureWarning: cols is deprecated, use columns

warnings.warn("cols is deprecated, use columns", FutureWarning)
<<<-------------------------------------------------------------------------

>>>-------------------------------------------------------------------------
Warning in /tmp/doc/source/comparison_with_r.rst at block ending on line 407
Specify :okwarning: as an option in the ipython:: block to suppress this message
----------------------------------------------------------------------------
/home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/pandas/tools/pivot.py:73: FutureWarning: rows is deprecated, use index

warnings.warn("rows is deprecated, use index", FutureWarning)
<<<-------------------------------------------------------------------------
```

Otherwise looking OK!
 ah, it is still from the pivot/pivot_table PR, but still needs to be updated.
 Btw - you can also explicitly set the warnings module run to raise on
warnings...
On Mar 22, 2014 1:04 AM, "Joris Van den Bossche" notifications@github.com
wrote:

> ah, it is still from the pivot/pivot_table PR, but still needs to be
> updated.
> 
> ## 
> 
> Reply to this email directly or view it on GitHubhttps://github.com/pydata/pandas/pull/6686#issuecomment-38345685
> .
 Fixed two noted issues in docs.  @jorisvandenbossche I couldn't find the result you pasted in any of the test jobs; how did you find that result?
 You would get the same if you build the docs yourself, but Travis also builds the docs, very handy! In the log of Travis, almost at the bottom, you have `$ if [ -f /tmp/doc.log ]; then cat /tmp/doc.log; fi`, where you get the output of the docs builds (you have to look at travis job 3, with `DOC_BUILD=true`)
 @jsexauer And you still missed 1 occurence of  `cols` and 2 of `rows` in comparison_with_r.rst (the docs only give one warning, because always only the first one is warned for)
 @jsexauer looks fine....pls create an issue to implement the removal of this code (reference this issue); i'll tag and put in 0.16.

@jorisvandenbossche @jtratner ok with this?
 @jsexauer #6680 has your name on it too!
 Just amended #6581, figured best keep it all in one ticket when we have to remove it all :)
 Closing issue.  Going to fix in #6693 instead.
