code: function getTarballStream (pkg, config, cb) {
    if(config.casDb && pkg.shasum) {
      var db = config.casDb
      db.has(pkg.shasum, function (err, stat) {
        if(!err)
          cb(null, db.getStream(pkg.shasum))
        else //fallback to the old way...
          tryCache()
      })
    }
    else
      tryCache()
  
    function tryCache () {
      var cache = getCache(pkg, config)
      fs.stat(cache, function (err) {
        if(!err)
          cb(null, fs.createReadStream(cache))
        else
          getDownload(pkg, config, cb)
      })
    }
  }
docs: stream a tarball - either from cache or registry