providing accumulator example as proposed in FLINK-828

- shows how to build a custom accumulator (for vectors)
- employ that accumulator for obtaining filter statistics
Why are you not using a `Tuple9<String,String, ...>` for the DataSet ?
This works also fine but you are loosing the compile-time type checking. Basically the typeparamters will make line 105: https://github.com/apache/incubator-flink/pull/55/files#diff-70cc9ad1c8f8e2e82718452620b0d33eR105 safer.

I think its fine in this case to use the `Tuple` but I want to note that.