[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 Quickstart example (by using the latest 0.5.2 Stratosphere release)
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
Merge branch 'master' of git://git.apache.org/incubator-flink into streaming-ready