Create json files for individual package descriptions

As mentined in another issue, I am interested in a JSON source with information about a specific package (author, desciption etc.). As suggested I hacked support for that into your tool.

It now creates a directory `packages` with one file for every package-version-combination. The content is what `Data.Aeson.TH` makes out of the Cabal data structures, not very beatiful, but good enough. It also creates `<pkg>.json` symlinks to the latest versions.

I did not invest much time to avoid additional work, i.e. it re-generates all files for now. Shout if that is a problem.

I have a strange problem with the code: Running it I get `hackage-jsonp: NoParse "(none)" 0`. But only if I don’t comment out the calls to `updateLink` – but why should that influence the tar parser (unless we are seeing some timeout or other effect). Can you reproduce this?