Paginate blog. Oh, and MOVE ALL THE THINGS \o|

Squashed most hacks where pages are stuff into <category>/_posts/YYYY-MM-DD-blah.md files.

* Proper pages were moved to places like <topic>/index.md or <topic>/<subtopic>.md
* All blog posty things are now in the root _posts/ directory
  * Exception: installations/ still has _posts dir so that the pages there show
    up in the blog.
Update README, drop old POSTING instructions