[SPARK-5871] output explain in Python

Author: Davies Liu <davies@databricks.com>

Closes apache#4658 from davies/explain and squashes the following commits:

db87ea2 [Davies Liu] output explain in Python

python/pyspark/sql/dataframe.py

@@ -244,8 +244,25 @@ def explain(self, extended=False):
         debugging purpose.
 
         If extended is False, only prints the physical plan.
-        """
-        self._jdf.explain(extended)
+
+        >>> df.explain()
+        PhysicalRDD [age#0,name#1], MapPartitionsRDD[...] at mapPartitions at SQLContext.scala:...
+
+        >>> df.explain(True)
+        == Parsed Logical Plan ==
+        ...
+        == Analyzed Logical Plan ==
+        ...
+        == Optimized Logical Plan ==
+        ...
+        == Physical Plan ==
+        ...
+        == RDD ==
+        """
+        if extended:
+            print self._jdf.queryExecution().toString()
+        else:
+            print self._jdf.queryExecution().executedPlan().toString()
 
     def isLocal(self):
         """
@@ -1034,7 +1051,7 @@ def _test():
                                   Row(name='Bob', age=5, height=85)]).toDF()
     (failure_count, test_count) = doctest.testmod(
         pyspark.sql.dataframe, globs=globs,
-        optionflags=doctest.ELLIPSIS | doctest.NORMALIZE_WHITESPACE)
+        optionflags=doctest.ELLIPSIS | doctest.NORMALIZE_WHITESPACE | doctest.REPORT_NDIFF)
     globs['sc'].stop()
     if failure_count:
         exit(-1)