createNewSparkContext should use sparkHome/jars/environment.

This fixes a bug introduced by Matei's recent change.

Very minor change in a testcase

Fixed bug in BlockManager and added a testcase

For some reason, ScalaDoc complained about no such constructor for
StreamingContext; it doesn't seem like an actual Scala error but it
prevented sbt publish and from working because docs weren't built.

Minor changes based on feedback

Also warns if spark.cleaner.ttl is not set in the version where you pass
your own SparkContext.

Some changes to streaming failure docs.

TD gave me the go-ahead to just make these changes:
- Define stateful dstream
- Some minor wording fixes

Disable remote lifecycle logging from Akka.