Updated JavaStreamingContext to make scaladoc compile.

`sbt/sbt doc` used to fail. This fixed it.
Merge pull request #410 from rxin/scaladoc1

Updated JavaStreamingContext to make scaladoc compile.

`sbt/sbt doc` used to fail. This fixed it.
Merge remote-tracking branch 'apache/master' into filestream-fix
Removed StreamingContext.registerInputStream and registerOutputStream - they were useless as InputDStream has been made to register itself. Also made DStream.register() private[streaming] - not useful to expose the confusing function. Updated a lot of documentation.