[FLINK-984] Compiler tests for distinct(). Adopted the old code from @markus-h

The code originates from https://github.com/apache/incubator-flink/pull/61.
The pull request also contained code to add POJO support to the distinct() operator. This has already been implemented in earlier work
[Docs] Fix rendering issue and typo
[FLINK-1292] Allow for longer normalized keys when using composite keys

This closes #241
[Java API] [docs] Rephrase Javadoc of minBy/maxBy and link from min/max

This closes #244.
FLINK-1272: Add a "reduceWithKey" function