[FLINK-828] Implement accumulator example

- shows how to build a custom accumulator (for vectors)
- employ that accumulator for obtaining filter statistics

This closes #55.
Reconfigure Apache Rat to enforce the correct ASF license header
Fix scope for scalatest dependency
Improve exception messages for file output format when target file cannot be created.
[FLINK-955] ResizingHashTable: automatic resizing, IndexOutOfBoundsException fixed, pointers

This closes #57