Updated link for pyspark examples in docs

Author: Jyotiska NK <jyotiska123@gmail.com>

Closes #22 from jyotiska/pyspark_docs and squashes the following commits:

426136c [Jyotiska NK] Updated link for pyspark examples
SPARK-1168, Added foldByKey to pyspark.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #115 from ScrapCodes/SPARK-1168/pyspark-foldByKey and squashes the following commits:

db6f67e [Prashant Sharma] SPARK-1168, Added foldByKey to pyspark.
Spark-1163, Added missing Python RDD functions

Author: prabinb <prabin.banka@imaginea.com>

Closes #92 from prabinb/python-api-rdd and squashes the following commits:

51129ca [prabinb] Added missing Python RDD functions Added __repr__ function to StorageLevel class. Added doctest for RDD.getStorageLevel().

Conflicts:

	python/pyspark/rdd.py
SPARK-977 Added Python RDD.zip function

was raised earlier as a part of  apache/incubator-spark#486

Author: Prabin Banka <prabin.banka@imaginea.com>

Closes #76 from prabinb/python-api-zip and squashes the following commits:

b1a31a0 [Prabin Banka] Added Python RDD.zip function

Conflicts:

	python/pyspark/rdd.py
SPARK-1240: handle the case of empty RDD when takeSample

https://spark-project.atlassian.net/browse/SPARK-1240

It seems that the current implementation does not handle the empty RDD case when run takeSample

In this patch, before calling sample() inside takeSample API, I add a checker for this case and returns an empty Array when it's a empty RDD; also in sample(), I add a checker for the invalid fraction value

In the test case, I also add several lines for this case

Author: CodingCat <zhunansjtu@gmail.com>

Closes #135 from CodingCat/SPARK-1240 and squashes the following commits:

fef57d4 [CodingCat] fix the same problem in PySpark
36db06b [CodingCat] create new test cases for takeSample from an empty red
810948d [CodingCat] further fix
a40e8fb [CodingCat] replace if with require
ad483fd [CodingCat] handle the case with empty RDD when take sample

Conflicts:
	core/src/main/scala/org/apache/spark/rdd/RDD.scala
	core/src/test/scala/org/apache/spark/rdd/RDDSuite.scala
SPARK-1244: Throw exception if map output status exceeds frame size

This is a very small change on top of @andrewor14's patch in #147.

Author: Patrick Wendell <pwendell@gmail.com>
Author: Andrew Or <andrewor14@gmail.com>

Closes #152 from pwendell/akka-frame and squashes the following commits:

e5fb3ff [Patrick Wendell] Reversing test order
393af4c [Patrick Wendell] Small improvement suggested by Andrew Or
8045103 [Patrick Wendell] Breaking out into two tests
2b4e085 [Patrick Wendell] Consolidate Executor use of akka frame size
c9b6109 [Andrew Or] Simplify test + make access to akka frame size more modular
281d7c9 [Andrew Or] Throw exception on spark.akka.frameSize exceeded + Unit tests
(cherry picked from commit 796977acdb5c96ca5c08591657137fb3e44d2e94)

Conflicts:

	core/src/test/scala/org/apache/spark/AkkaUtilsSuite.scala
	core/src/test/scala/org/apache/spark/MapOutputTrackerSuite.scala
[Spark-1261] add instructions for running python examples to doc overview page

Author: Diana Carroll <dcarroll@cloudera.com>

Closes #162 from dianacarroll/SPARK-1261 and squashes the following commits:

14ac602 [Diana Carroll] typo in python example text
5121e3e [Diana Carroll] Add explanation of how to run Python examples to main doc overview page
Merge branch 'branch-0.9' of github.com:apache/spark into csd-2.10