Fixes FLINK-1276

Closes #229
[FLINK-1249] [APIs] [compiler] Add custom partitioner for CoGroup

This closes #228.
[FLINK-1221] Use only method name and location as default operator names
[FLINK-1221] Use StackTraceElement methods instead of String parsing

This closes #231.
Small changes and tests for the newly introduced EnumType

This closes #230.
FLINK-1272: Add a "reduceWithKey" function