Added doctest for map function in rdd.py

Doctest added for map in rdd.py

Author: Jyotiska NK <jyotiska123@gmail.com>

Closes #177 from jyotiska/pyspark_rdd_map_doctest and squashes the following commits:

a38527f [Jyotiska NK] Added doctest for map function in rdd.py

python/pyspark/rdd.py

@@ -180,6 +180,10 @@ class RDD(object):
     def map(self, f, preservesPartitioning=False):
         """
         Return a new RDD by applying a function to each element of this RDD.
+        
+        >>> rdd = sc.parallelize(["b", "a", "c"])
+        >>> sorted(rdd.map(lambda x: (x, 1)).collect())
+        [('a', 1), ('b', 1), ('c', 1)]
         """
         def func(split, iterator): return imap(f, iterator)
         return PipelinedRDD(self, func, preservesPartitioning)