Use RDD type for `transform` operator in Java.

This is an improved implementation of the `transform` operator in Java.
The main difference is that this allows all four possible types of
transform functions

1. JavaRDD -> JavaRDD
2. JavaRDD -> JavaPairRDD
3. JavaPairRDD -> JavaPairRDD
4. JavaPairRDD -> JavaRDD

whereas previously only (1) and (3) were possible.

Conflicts:

	streaming/src/test/java/spark/streaming/JavaAPISuite.java