code: function skip() {
      rimraf(file, function() {})
      delete f.versions[v]
      if (f._attachments)
        delete f._attachments[file]
      need.splice(need.indexOf(v), 1)
      maybeDone(null)
    }
docs: TODO: If the file already exists, get its size. If the size matches content-length, get the md5 If the md5 matches content-md5, then don't bother downloading!