[SPARK-5457][SQL] Add missing DSL for ApproxCountDistinct.

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

Closes #4250 from ueshin/issues/SPARK-5457 and squashes the following commits:

3c05e59 [Takuya UESHIN] Remove parameter to use default value of ApproxCountDistinct.
faea19d [Takuya UESHIN] Use overload instead of default value for Java support.
d1cca38 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-5457
663d43d [Takuya UESHIN] Add missing DSL for ApproxCountDistinct.

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

@@ -105,6 +105,11 @@ object Dsl {
   def countDistinct(expr: Column, exprs: Column*): Column =
     CountDistinct((expr +: exprs).map(_.expr))
 
+  def approxCountDistinct(e: Column): Column =
+    ApproxCountDistinct(e.expr)
+  def approxCountDistinct(e: Column, rsd: Double): Column =
+    ApproxCountDistinct(e.expr, rsd)
+
   def avg(e: Column): Column = Average(e.expr)
   def first(e: Column): Column = First(e.expr)
   def last(e: Column): Column = Last(e.expr)