Moved DStream, DStreamCheckpointData and PairDStream from org.apache.spark.streaming to org.apache.spark.streaming.dstream.
Merge branch 'error-handling' into dstream-move
Merge remote-tracking branch 'apache/master' into dstream-move
Fixed persistence logic of WindowedDStream, and fixed default persistence level of input streams.
Merge remote-tracking branch 'apache/master' into dstream-move

Conflicts:
	streaming/src/main/scala/org/apache/spark/streaming/dstream/DStream.scala
Fixed import formatting.
Merge pull request #400 from tdas/dstream-move

Moved DStream and PairDSream to org.apache.spark.streaming.dstream

Similar to the package location of `org.apache.spark.rdd.RDD`, `DStream` has been moved from `org.apache.spark.streaming.DStream` to `org.apache.spark.streaming.dstream.DStream`. I know that the package name is a little long, but I think its better to keep it consistent with Spark's structure.

Also fixed persistence of windowed DStream. The RDDs generated generated by windowed DStream are essentially unions of underlying RDDs, and persistent these union RDDs would store numerous copies of the underlying data. Instead setting the persistence level on the windowed DStream is made to set the persistence level of the underlying DStream.