Add yesod-auth-hashdb to platform and Yesod.Auth description

Michael - as we discussed earlier, yesod-auth-hashdb is now on hackage.  As well as adding it to the platform in this pull request, I've changed the auth plugins description in yesod-auth.cabal.  Not sure whether the version number change in the platform is what you want, but I guess it will change for other reasons anyway.