on(uri, callback) {
    if (this._groupData !== null) {
      if (this._groupData.prefix !== undefined) {
        uri = this._groupData.prefix + uri;
      }
    }
    uri = this.modifyURI(uri);

    if (this._routes[uri] === undefined) {
      this._routes[uri] = [];
    }
    this._routes[uri].push(callback);
  }
Define new route

@param {string} uri
@param {callback|Function} callback