Throw exception if task result exceeds Akka frame size.

This partially addresses SPARK-747.
Abort job if result exceeds Akka frame size; add test.
Fix running on yarn documentation
Filter out nulls - prevent NPE
Fix documentation to use yarn-standalone as master
Hope this does not turn into a bike shed change
Updated Scala version in docs generation ruby script.
Fix example jar name
Merge branch 'master' of github.com:mesos/spark
Merge branch 'master' of https://github.com/mridulm/spark
Fixed a couple typos and formating problems in the YARN documentation.
Changed the logging level from info to warning when addJar(null) is
called.
Merge pull request #610 from JoshRosen/spark-747

Throw exception if TaskResult exceeds Akka frame size
DistributedSuite: remove dead test code
Merge pull request #618 from woggling/dead-code-disttest

DistributedSuite: remove dead code
Use ARRAY_SAMPLE_SIZE constant instead of 100.0
Merge pull request #619 from woggling/adjust-sampling

Use ARRAY_SAMPLE_SIZE constant instead of hard-coded 100.0 in SizeEstimator
Add Python timing instrumentation
Prefork Python worker processes
Add tests and fixes for Python daemon shutdown