Initial implementation of rolling iterators
- [x] closes #11704
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hi, I have yet to implements tests, use flake8 and create a whatsnew entry as I am uncertain whether this implementation would be correct.

I would really appreciate it if someone could give me some feedback on the code.

This PR will implement iterators for window functions.

Some questions I have:

* Does DataFrame.rolling(x) yield DataFrames?
* Does Series.rolling(x) yield Series?
Hello @ThomasKluiters! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-07-21 11:48:05 UTC Can you add tests for this? Also I think yes to both of your questions > Can you add tests for this?

I wouldn't mind adding tests! I had to know what the expected output would be, but now that I know I'll be writing them! Thanks :) @mroeschke I have changed the files according to your feedback, could you have a look? > Can you add `expanding` to the tests here as well?

They're already there, view the changes under test_expanding :) @ThomasKluiters thanks - sorry misread that on my end > Looking good. Could you also add tests where `min_periods` is utilized?

@mroeschke 
Expanding kind of already does this, but I have added three new unit tests that use window and min_periods, please check! For the `min_periods` tests, could you also test where there's `nan`s in the Series/DataFrames? > For the `min_periods` tests, could you also test where there's `nan`s in the Series/DataFrames?

Well, it turns out that this required a bit of work - I've added some tests for it and had to change some code in window.py @jreback 

I have moved the tests to `test_iterator` and added the reference to the issue at the top.
I have moved the whatsnew to 1.0.0.
I have added the documentation to computation.rst

Though, I can't seem to figure out how to reference computation.rst in the Whatsnew, could you help me with this please :)? Thanks!
 can you merge master and see if can get passing IIRC this looked reasonable, can you merge master and update to any comments closing this as stale, though its a nice change, needs a rebase and comments addressed.