Refactored streaming scheduler and added listener interface.

- Refactored Scheduler + JobManager to JobGenerator + JobScheduler and
  added JobSet for cleaner code. Moved scheduler related code to
  streaming.scheduler package.
- Added StreamingListener trait (similar to SparkListener) to enable
  gathering to streaming stats like processing times and delays.
  StreamingContext.addListener() to added listeners.
- Deduped some code in streaming tests by modifying TestSuiteBase, and
  added StreamingListenerSuite.
Added StatsReportListener to generate processing time statistics across multiple batches.
Merge branch 'apache-master' into scheduler-update

Conflicts:
	streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala
	streaming/src/main/scala/org/apache/spark/streaming/dstream/ForEachDStream.scala
Minor changes.
Minor updated based on comments on PR 277.
Added comments to BatchInfo and JobSet, based on Patrick's comment on PR 277.
Minor formatting fixes.
Minor change for PR 277.
Merge pull request #277 from tdas/scheduler-update

Refactored the streaming scheduler and added StreamingListener interface

- Refactored the streaming scheduler for cleaner code. Specifically, the JobManager was renamed to JobScheduler, as it does the actual scheduling of Spark jobs to the SparkContext. The earlier Scheduler was renamed to JobGenerator, as it actually generates the jobs from the DStreams. The JobScheduler starts the JobGenerator. Also, moved all the scheduler related code from spark.streaming to spark.streaming.scheduler package.
- Implemented the StreamingListener interface, similar to SparkListener. The streaming version of StatusReportListener prints the batch processing time statistics (for now). Added StreamingListernerSuite to test it.
- Refactored streaming TestSuiteBase for deduping code in the other streaming testsuites.