providing accumulator example as proposed in FLINK-828

- shows how to build a custom accumulator (for vectors)
- employ that accumulator for obtaining filter statistics
Can you remove the "(non-Javadoc)" javadoc comments? They don't add any value, because its obvious that there is no Javadoc.

We had quite a lot of these comments in our project in the past and did a huge search/replace session to remove all of them.

As a side note: It seems that you've limited with width of your code. We are currently not limiting the width, so you don't need to wrap long lines.