[SPARK-7776] [STREAMING] Added shutdown hook to StreamingContext

Shutdown hook to stop SparkContext was added recently. This results in ugly errors when a streaming application is terminated by ctrl-C.

```
Exception in thread "Thread-27" org.apache.spark.SparkException: Job cancelled because SparkContext was shut down
	at org.apache.spark.scheduler.DAGScheduler$$anonfun$cleanUpAfterSchedulerStop$1.apply(DAGScheduler.scala:736)
	at org.apache.spark.scheduler.DAGScheduler$$anonfun$cleanUpAfterSchedulerStop$1.apply(DAGScheduler.scala:735)
	at scala.collection.mutable.HashSet.foreach(HashSet.scala:79)
	at org.apache.spark.scheduler.DAGScheduler.cleanUpAfterSchedulerStop(DAGScheduler.scala:735)
	at org.apache.spark.scheduler.DAGSchedulerEventProcessLoop.onStop(DAGScheduler.scala:1468)
	at org.apache.spark.util.EventLoop.stop(EventLoop.scala:84)
	at org.apache.spark.scheduler.DAGScheduler.stop(DAGScheduler.scala:1403)
	at org.apache.spark.SparkContext.stop(SparkContext.scala:1642)
	at org.apache.spark.SparkContext$$anonfun$3.apply$mcV$sp(SparkContext.scala:559)
	at org.apache.spark.util.SparkShutdownHook.run(Utils.scala:2266)
	at org.apache.spark.util.SparkShutdownHookManager$$anonfun$runAll$1$$anonfun$apply$mcV$sp$1.apply$mcV$sp(Utils.scala:2236)
	at org.apache.spark.util.SparkShutdownHookManager$$anonfun$runAll$1$$anonfun$apply$mcV$sp$1.apply(Utils.scala:2236)
	at org.apache.spark.util.SparkShutdownHookManager$$anonfun$runAll$1$$anonfun$apply$mcV$sp$1.apply(Utils.scala:2236)
	at org.apache.spark.util.Utils$.logUncaughtExceptions(Utils.scala:1764)
	at org.apache.spark.util.SparkShutdownHookManager$$anonfun$runAll$1.apply$mcV$sp(Utils.scala:2236)
	at org.apache.spark.util.SparkShutdownHookManager$$anonfun$runAll$1.apply(Utils.scala:2236)
	at org.apache.spark.util.SparkShutdownHookManager$$anonfun$runAll$1.apply(Utils.scala:2236)
	at scala.util.Try$.apply(Try.scala:161)
	at org.apache.spark.util.SparkShutdownHookManager.runAll(Utils.scala:2236)
	at org.apache.spark.util.SparkShutdownHookManager$$anon$6.run(Utils.scala:2218)
	at org.apache.hadoop.util.ShutdownHookManager$1.run(ShutdownHookManager.java:54)
```

This is because the Spark's shutdown hook stops the context, and the streaming jobs fail in the middle. The correct solution is to stop the streaming context before the spark context. This PR adds the shutdown hook to do so with a priority higher than the SparkContext's shutdown hooks priority.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #6307 from tdas/SPARK-7776 and squashes the following commits:

e3d5475 [Tathagata Das] Added conf to specify graceful shutdown
4c18652 [Tathagata Das] Added shutdown hook to StreamingContxt.