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.
This is should be outside of try, otherwise you may get NPE in the finally block. It should be:

```java
BufferReader reader = new BufferedReader(...);
try {
  // Use the reader
} finally {
  Closeables.closeQuietly(reader);
}
```