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

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

Changed networkStream to socketStream and pluggableNetworkStream to become networkStream as a way to create streams from arbitrary network receiver.

Added checkpointing and fault-tolerance semantics to the programming guide. Fixed default checkpoint interval to being a multiple of slide duration. Fixed visibility of some classes and objects to clean up docs.

Only brand new RDDs (e.g. parallelize and makeRDD) now use default
parallelism, everything else uses their largest parent's partitioner
or partition size.

Conflicts:
	docs/_config.yml

ordering, so the reduce function must also be commutative.

Also, adds a line in doc explaining how to use.

It's nicer if all the commands you need are made explicit.

Made StorageLevel constructor private, and added StorageLevels.create() to the Java API. Updates scala and java programming guides.