Recent change in layout of PG Docs broke my spider for why-upgrade.depesz.com.

Today got some time and decided to bite the bullet.

Fixed spider code, used it to get new changelog, and while I was at it, did couple of slight modifications of the site:

• display count of all changes that are there in given upgrade path (in section header)
• fixed a bug that caused empty Security fixes: box to display, even when there are no security fixes.
• added metainfo page, which shows some stats about why-upgrade “database".

All in all, not big changes, but I hope you'll find it useful.