Chase changes in Hackage.

Note that it does not export the full upload log any more, so it is
pre-generated from index.tar.gz (by hsmtk/get_hackage.log.sh) and
maintained separately on haskell.inf.elte.hu.

Inspired by:  A pull request from Tim Matthews
Minor refactor.
Allow pruning available updates by constraints of a specific package.
Update contents of README.