Revert "Disable POJO types"

This reverts commit 1637cb12500cacfd4fdd9b0112c7af1683768e84.

Conflicts:
	flink-examples/flink-java-examples/src/main/java/org/apache/flink/example/java/wordcount/WordCountPOJO.java
Revert "Remove expression key methods"

This reverts commit b63beba56fc995dba87d6fc4766af2900c0438d5.

Conflicts:
	flink-examples/flink-java-examples/src/main/java/org/apache/flink/example/java/wordcount/WordCountPOJO.java
[FLINK-1032] Allow Access of Nested Fields in POJO Field Expressions

This extends the PojoTypeInfo to allow key expressions like "foo.bar".
Right now the implementation is not very pretty but without turning to
code generation this is the way to do it, IMHO.
Fix bug with using POJO type as return value of key selector function

Also fix using field expressions for ReduceGroup Operator