Add subtractByKey to the JavaPairRDD wrapper

Author: Punya Biswal <pbiswal@palantir.com>

Closes #600 from punya/subtractByKey-java and squashes the following commits:

e961913 [Punya Biswal] Hide implicit ClassTags from Java API
c5d317b [Punya Biswal] Add subtractByKey to the JavaPairRDD wrapper

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

@@ -277,6 +277,29 @@ class JavaPairRDD[K, V](val rdd: RDD[(K, V)])(implicit val kClassTag: ClassTag[K
   def subtract(other: JavaPairRDD[K, V], p: Partitioner): JavaPairRDD[K, V] =
     fromRDD(rdd.subtract(other, p))
 
+  /**
+   * Return an RDD with the pairs from `this` whose keys are not in `other`.
+   *
+   * Uses `this` partitioner/partition size, because even if `other` is huge, the resulting
+   * RDD will be <= us.
+   */
+  def subtractByKey[W](other: JavaPairRDD[K, W]): JavaPairRDD[K, V] = {
+    implicit val cmw: ClassTag[W] = implicitly[ClassTag[AnyRef]].asInstanceOf[ClassTag[W]]
+    fromRDD(rdd.subtractByKey(other))
+  }
+
+  /** Return an RDD with the pairs from `this` whose keys are not in `other`. */
+  def subtractByKey[W](other: JavaPairRDD[K, W], numPartitions: Int): JavaPairRDD[K, V] = {
+    implicit val cmw: ClassTag[W] = implicitly[ClassTag[AnyRef]].asInstanceOf[ClassTag[W]]
+    fromRDD(rdd.subtractByKey(other, numPartitions))
+  }
+
+  /** Return an RDD with the pairs from `this` whose keys are not in `other`. */
+  def subtractByKey[W](other: JavaPairRDD[K, W], p: Partitioner): JavaPairRDD[K, V] = {
+    implicit val cmw: ClassTag[W] = implicitly[ClassTag[AnyRef]].asInstanceOf[ClassTag[W]]
+    fromRDD(rdd.subtractByKey(other, p))
+  }
+
   /**
    * Return a copy of the RDD partitioned using the specified partitioner.
    */