HOTFIX: Add no-arg SparkContext constructor in Java

Self explanatory.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #878 from pwendell/java-constructor and squashes the following commits:

2cc1605 [Patrick Wendell] HOTFIX: Add no-arg SparkContext constructor in Java

core/src/main/scala/org/apache/spark/api/java/JavaSparkContext.scala

@@ -41,6 +41,12 @@ import org.apache.spark.rdd.RDD
  * [[org.apache.spark.api.java.JavaRDD]]s and works with Java collections instead of Scala ones.
  */
 class JavaSparkContext(val sc: SparkContext) extends JavaSparkContextVarargsWorkaround {
+  /**
+   * Create a JavaSparkContext that loads settings from system properties (for instance, when
+   * launching with ./bin/spark-submit).
+   */
+  def this() = this(new SparkContext())
+
   /**
    * @param conf a [[org.apache.spark.SparkConf]] object specifying Spark parameters
    */