Occured when calling nextKey() and iterator not consumed with *last* key group.

Implemented JoinOperatorBase and test cases.

Make groupReduce code compliant with pre-java-8 versions, fix java8 tests with moved type information classes.

Fix Various warnings.

Also fix some bugs resulting from moving stuff between packages

  - Remove class loader (was inconsistently used and set)
  - Objects are stored in their type, rather than as a string

Is now in JoinOperator and CoGroupOperator. We don't need
the special Scala API sanity check anymore now since they
use java operators that now correctly check in the base
class.

This closes #137

[streaming] ConnectedDataStream API  and operator cleanup + modified windowReduceGroup functionality

[streaming] Added CoBatchReduceInvokable, CoWindowReduceInvokable, CoGroupedBatchReduceInvokable, CoGroupedWindowReduceInvokable