[FLINK-1032] Rework support for POJO types in the Java API
Added expression keys to distinct and partition operator and addressed some of the comments
Add Pojo support to Scala API
Various fixes
- improved error messages
- the composite vs atomic bug aljoscha found
- Comparator test for Pojo Comparator enabled
- TODO removed
- string-based key expression for group sorting fields definition
- support for specifying "select all" using * and now also _ (for scala fans)
- Exception if user is having multiple fields with the same name in the class
Documentation and fixes
Really add POJO support and nested keys for Scala API

This also adds more integration tests, but not all tests of the Java API
have been ported to Scala yet.