[SPARK-5551][SQL] Create type alias for SchemaRDD for source backward compatibility

Author: Reynold Xin <rxin@databricks.com>

Closes #4327 from rxin/schemarddTypeAlias and squashes the following commits:

e5a8ff3 [Reynold Xin] [SPARK-5551][SQL] Create type alias for SchemaRDD for source backward compatibility

sql/core/src/main/scala/org/apache/spark/sql/package.scala

@@ -38,4 +38,10 @@ package object sql {
    */
   @DeveloperApi
   protected[sql] type Strategy = org.apache.spark.sql.catalyst.planning.GenericStrategy[SparkPlan]
+
+  /**
+   * Type alias for [[DataFrame]]. Kept here for backward source compatibility for Scala.
+   */
+  @deprecated("1.3.0", "use DataFrame")
+  type SchemaRDD = DataFrame
 }