DOC: Change release and whatsnew
Some cleanup & changes to facilitate release automation

* We will include the correct (latest on master or maintenance branch)
  whatsnew directly in the index.rst toctree
* Contributors are included in the whatsnew for each version (automatically)
* Removed release.rst
* Added new releases.rst which has toctrees for each release
![screen shot 2018-06-22 at 3 10 23 pm](https://user-images.githubusercontent.com/1312546/41797116-6bfe43aa-762e-11e8-840f-9639faf14e8b.png)

Incidental changes

* Updated style.ipynb. Writing the jinja template was confusing sphinx. We
  included it in the git source now.
* Fixing some inconsitent header levels (will do more)
* Refactored announce.py to support auto-generated contributors

TODO:

- [x] Finish up the rest of the whatsnews

cc @jorisvandenbossche, @datapythonista