Added new Spark Streaming operations

New operations 
- transformWith which allows arbitrary 2-to-1 DStream transform, added to Scala and Java API
- StreamingContext.transform to allow arbitrary n-to-1 DStream
- leftOuterJoin and rightOuterJoin between 2 DStreams, added to Scala and Java API
- missing variations of join and cogroup added to Scala Java API
- missing JavaStreamingContext.union
- Updated a number of Java and Scala API docs