Made Serializer and JavaSerializer non private.

core/src/main/scala/spark/JavaSerializer.scala

@@ -57,6 +57,6 @@ private[spark] class JavaSerializerInstance extends SerializerInstance {
   }
 }
 
-private[spark] class JavaSerializer extends Serializer {
+class JavaSerializer extends Serializer {
   def newInstance(): SerializerInstance = new JavaSerializerInstance
 }

core/src/main/scala/spark/Serializer.scala

@@ -12,7 +12,7 @@ import spark.util.ByteBufferInputStream
  * A serializer. Because some serialization libraries are not thread safe, this class is used to
  * create SerializerInstances that do the actual serialization.
  */
-private[spark] trait Serializer {
+trait Serializer {
   def newInstance(): SerializerInstance
 }