[SPARK-1778] [SQL] Add 'limit' transformation to SchemaRDD.

Add `limit` transformation to `SchemaRDD`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #711 from ueshin/issues/SPARK-1778 and squashes the following commits:

33169df [Takuya UESHIN] Add 'limit' transformation to SchemaRDD.

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

@@ -178,6 +178,15 @@ class SchemaRDD(
   def orderBy(sortExprs: SortOrder*): SchemaRDD =
     new SchemaRDD(sqlContext, Sort(sortExprs, logicalPlan))
 
+  /**
+   * Limits the results by the given expressions.
+   * {{{
+   *   schemaRDD.limit(10)
+   * }}}
+   */
+  def limit(limitExpr: Expression): SchemaRDD =
+    new SchemaRDD(sqlContext, Limit(limitExpr, logicalPlan))
+
   /**
    * Performs a grouping followed by an aggregation.
    *

sql/core/src/test/scala/org/apache/spark/sql/DslQuerySuite.scala

@@ -71,6 +71,12 @@ class DslQuerySuite extends QueryTest {
       Seq((3,1), (3,2), (2,1), (2,2), (1,1), (1,2)))
   }
 
+  test("limit") {
+    checkAnswer(
+      testData.limit(10),
+      testData.take(10).toSeq)
+  }
+
   test("average") {
     checkAnswer(
       testData2.groupBy()(Average('a)),