[JDBC] WIP: draft release notes for 42.1.2

I've pushed draft release notes for 42.1.2.

Please feel free to review it at https://github.com/pgjdbc/pgjdbc/pull/864  (see docs/_posts/2017-07-11-42.1.2-release.md)

Note: I did refactor site generation a bit, so it be way easier to cut a new release. Basically, it builds index, download and changelog page out of _posts folder. That is just one file (with new release notes) is enough to update all the pages.

Note: the side effect is jekyll creates /new_release/2017/05/04/42.1.0-release.html pages, however those are never linked. I think it should not harm much.

Dave, could you please verify if you are able to build the site out of my branch?

Vladimir