[SPARK-2079] Support batching when serializing SchemaRDD to Python

Added batching with default batch size 10 in SchemaRDD.javaToPython

Author: Kan Zhang <kzhang@apache.org>

Closes #1023 from kanzhang/SPARK-2079 and squashes the following commits:

2d1915e [Kan Zhang] [SPARK-2079] Add batching in SchemaRDD.javaToPython
19b0c09 [Kan Zhang] [SPARK-2079] Removing unnecessary wrapping in SchemaRDD.javaToPython

(cherry picked from commit 2550533a28382664f8fd294b2caa494d12bfc7c1)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2013] Documentation for saveAsPickleFile and pickleFile in Python

Author: Kan Zhang <kzhang@apache.org>

Closes #983 from kanzhang/SPARK-2013 and squashes the following commits:

0e128bb [Kan Zhang] [SPARK-2013] minor update
e728516 [Kan Zhang] [SPARK-2013] Documentation for saveAsPickleFile and pickleFile in Python

(cherry picked from commit b52603b039cdfa0f8e58ef3c6229d79e732ffc58)
Signed-off-by: Reynold Xin <rxin@apache.org>

Conflicts:
	docs/programming-guide.md