code: function (identifierUrn, params) {
      var path = this.constructPath(constants.IDENTIFIERS, identifierUrn);
      return this.Core.GET(path, params);
    }
docs: Get all the identifiers or a particular identifier identified by the `identifierUrn` parameter
  @param  {String|Integer|undefined} identifierUrn A Robin identifier URN
  @param  {Object|undefined}         params        A querystring object
  @return {Function}                               A Promise