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
[SPARK-972] Added detailed callsite info for ValueError in context.py (resubmitted)

Author: jyotiska <jyotiska123@gmail.com>

Closes #34 from jyotiska/pyspark_code and squashes the following commits:

c9439be [jyotiska] replaced dict with namedtuple
a6bf4cd [jyotiska] added callsite info for context.py
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-1205: Clean up callSite/origin/generator.

This patch removes the `generator` field and simplifies + documents
the tracking of callsites.

There are two places where we care about call sites, when a job is
run and when an RDD is created. This patch retains both of those
features but does a slight refactoring and renaming to make things
less confusing.

There was another feature of an rdd called the `generator` which was
by default the user class that in which the RDD was created. This is
used exclusively in the JobLogger. It been subsumed by the ability
to name a job group. The job logger can later be refectored to
read the job group directly (will require some work) but for now
this just preserves the default logged value of the user class.
I'm not sure any users ever used the ability to override this.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #106 from pwendell/callsite and squashes the following commits:

fc1d009 [Patrick Wendell] Compile fix
e17fb76 [Patrick Wendell] Review feedback: callSite -> creationSite
62e77ef [Patrick Wendell] Review feedback
576e60b [Patrick Wendell] SPARK-1205: Clean up callSite/origin/generator.
SPARK-1211. In ApplicationMaster, set spark.master system property to "y...

...arn-cluster"

Author: Sandy Ryza <sandy@cloudera.com>

Closes #118 from sryza/sandy-spark-1211 and squashes the following commits:

d4001c7 [Sandy Ryza] SPARK-1211. In ApplicationMaster, set spark.master system property to "yarn-cluster"