SPARK-1815. SparkContext should not be marked DeveloperApi

Author: Sandy Ryza <sandy@cloudera.com>

Closes #753 from sryza/sandy-spark-1815 and squashes the following commits:

957a8ac [Sandy Ryza] SPARK-1815. SparkContext should not be marked DeveloperApi

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -50,7 +50,6 @@ import org.apache.spark.ui.SparkUI
 import org.apache.spark.util.{ClosureCleaner, MetadataCleaner, MetadataCleanerType, TimeStampedWeakValueHashMap, Utils}
 
 /**
- * :: DeveloperApi ::
  * Main entry point for Spark functionality. A SparkContext represents the connection to a Spark
  * cluster, and can be used to create RDDs, accumulators and broadcast variables on that cluster.
  *
@@ -58,7 +57,6 @@ import org.apache.spark.util.{ClosureCleaner, MetadataCleaner, MetadataCleanerTy
  *   this config overrides the default configs as well as system properties.
  */
 
-@DeveloperApi
 class SparkContext(config: SparkConf) extends Logging {
 
   // This is used only by YARN for now, but should be relevant to other cluster types (Mesos,