[SPARK-1161] Add saveAsPickleFile and SparkContext.pickleFile in Python

Author: Kan Zhang <kzhang@apache.org>

Closes #755 from kanzhang/SPARK-1161 and squashes the following commits:

24ed8a2 [Kan Zhang] [SPARK-1161] Fixing doc tests
44e0615 [Kan Zhang] [SPARK-1161] Adding an optional batchSize with default value 10
d929429 [Kan Zhang] [SPARK-1161] Add saveAsObjectFile and SparkContext.objectFile in Python