Added Java 8 to README.md
Moved SerializerTestBase to s-core and resolved Maven dependencies
GroupReduce testcases for the new java api
Minor improvements on GroupReduceITCase (tests for group sorting)
Testcases for cross operator in new java API
Minor improvements of CrossITCase + some clean-up
Mixed key types for cogroup
Fixed mixed keys for CoGroup, adapted test cases, code clean up