code: function sign(key, secret, uri, body, options) {
      // add timestamp
      uri.addQuery("timestamp", utils.createTimestamp());
  
      // add signature
      addSignature(key, secret, uri, body, options);
  
      return uri;
  }
docs: Sign a URI instance.
  Automatically adds a timestamp to the URI instance.
  
  @public
  
  @param {String} key - key used to sign
  @param {String} secret - used to sign the request
  @param {URIjs} uri - an instance of URIjs
  @param {*} body - body to used in request
  @param {Object} [options] - options
  @param {Number} [options.apiVersion] - API version
  @return {URIjs} same instance of URIjs, i.e. 'uri'
  @throws {errors.AuthenticationRequiredError} if 'key' or 'secret' are invalid.