[SPARK-4806] Streaming doc update for 1.2

Important updates to the streaming programming guide
- Make the fault-tolerance properties easier to understand, with information about write ahead logs
- Update the information about deploying the spark streaming app with information about Driver HA
- Update Receiver guide to discuss reliable vs unreliable receivers.
Maybe a missing word here; how about "if the receiver's configured storage level uses replication (which is enabled by default), ..."