[SPARK-7976] Add style checker to disallow overriding finalize.

Author: Reynold Xin <rxin@databricks.com>

Closes #6528 from rxin/style-finalizer and squashes the following commits:

a2211ca [Reynold Xin] [SPARK-7976] Enable NoFinalizeChecker.

core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala

@@ -843,6 +843,7 @@ private class PythonAccumulatorParam(@transient serverHost: String, serverPort:
  * An Wrapper for Python Broadcast, which is written into disk by Python. It also will
  * write the data into disk after deserialization, then Python can read it from disks.
  */
+// scalastyle:off no.finalize
 private[spark] class PythonBroadcast(@transient var path: String) extends Serializable {
 
   /**
@@ -884,3 +885,4 @@ private[spark] class PythonBroadcast(@transient var path: String) extends Serial
     }
   }
 }
+// scalastyle:on no.finalize

scalastyle-config.xml

@@ -96,7 +96,7 @@
  <!-- <check level="error" class="org.scalastyle.scalariform.ReturnChecker" enabled="true"></check> -->
  <!-- <check level="error" class="org.scalastyle.scalariform.NullChecker" enabled="true"></check> -->
  <!-- <check level="error" class="org.scalastyle.scalariform.NoCloneChecker" enabled="true"></check> -->
- <!-- <check level="error" class="org.scalastyle.scalariform.NoFinalizeChecker" enabled="true"></check> -->
+  <check level="error" class="org.scalastyle.scalariform.NoFinalizeChecker" enabled="true"></check>
  <!-- <check level="error" class="org.scalastyle.scalariform.CovariantEqualsChecker" enabled="true"></check> -->
  <!-- <check level="error" class="org.scalastyle.scalariform.StructuralTypeChecker" enabled="true"></check> -->
  <!-- <check level="error" class="org.scalastyle.file.RegexChecker" enabled="true"> -->