tools: add release tool and docs, remove old tools

Also added binary download documentation to the README.md and
GPG release key fingerprint for @rvagg.

PR-URL: https://github.com/iojs/io.js/pull/681


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>