SPARK-1833 - Have an empty SparkContext constructor.

This is nicer than relying on new SparkContext(new SparkConf())

Author: Patrick Wendell <pwendell@gmail.com>

Closes #774 from pwendell/spark-context and squashes the following commits:

ef9f12f [Patrick Wendell] SPARK-1833 - Have an empty SparkContext constructor.

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

@@ -66,6 +66,12 @@ class SparkContext(config: SparkConf) extends Logging {
   // contains a map from hostname to a list of input format splits on the host.
   private[spark] var preferredNodeLocationData: Map[String, Set[SplitInfo]] = Map()
 
+  /**
+   * Create a SparkContext that loads settings from system properties (for instance, when
+   * launching with ./bin/spark-submit).
+   */
+  def this() = this(new SparkConf())
+
   /**
    * :: DeveloperApi ::
    * Alternative constructor for setting preferred locations where Spark will create executors.