Spark Integration: SparkPageRank now uses Service

Added a simple service to SparkPageRank example application to showcase how to use services in Spark.

@anew : Please review. Thanks.
Doesn't look like a requirement from the documentation. Just sending an error message.

  /**
   * Sends an error message back to the client with the specified status code.
   *
   * @param status status of the HTTP response
   * @param errorMessage error message sent back to the client
   */