[SPARK-8967] [DOC] add Since annotation

Add `Since` as a Scala annotation. The benefit is that we can use it without having explicit JavaDoc. This is useful for inherited methods. The limitation is that is doesn't show up in the generated Java API documentation. This might be fixed by modifying genjavadoc. I think we could leave it as a TODO.

This is how the generated Scala doc looks:

`since` JavaDoc tag:

![screen shot 2015-08-11 at 10 00 37 pm](https://cloud.githubusercontent.com/assets/829644/9230761/fa72865c-40d8-11e5-807e-0f3c815c5acd.png)

`Since` annotation:

![screen shot 2015-08-11 at 10 00 28 pm](https://cloud.githubusercontent.com/assets/829644/9230764/0041d7f4-40d9-11e5-8124-c3f3e5d5b31f.png)

rxin

Author: Xiangrui Meng <meng@databricks.com>

Closes #8131 from mengxr/SPARK-8967.

core/src/main/scala/org/apache/spark/annotation/Since.scala

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.annotation
+
+import scala.annotation.StaticAnnotation
+
+/**
+ * A Scala annotation that specifies the Spark version when a definition was added.
+ * Different from the `@since` tag in JavaDoc, this annotation does not require explicit JavaDoc and
+ * hence works for overridden methods that inherit API documentation directly from parents.
+ * The limitation is that it does not show up in the generated Java API documentation.
+ */
+private[spark] class Since(version: String) extends StaticAnnotation