[FLINK-1070] Change return type of "getBroadcastVariable()" to List.

This closes #105
[FLINK-1075] Removed the AsynchronousPartialSorter.

This closes #104
[FLINK-1079] Fix inconsistent parameter naming
[FLINK-1078] PrintingOutputFormat uses same partition indexing as FileOutputFormat.
[FLINK-1084] Fix broken links in "How to add an operator"
Add Pi approximation Java example

This closes #78
Some simplifications to the PiEstimation example
[FLINK-1020] Introduce minBy and maxBy

This closes #101
[FLINK-610] Added KryoSerializer

This closes #74
Default generic serializer is Avro. Minor improvements in KryoSerializer. Add mini benchmark for Avro/Kryo.