Remove activation of profiles by default

See the discussion at https://github.com/mesos/spark/pull/355 for why
default profile activation is a problem.
Add more private declarations.
Changed PartitionPruningRDD's split to make sure it returns the correct
split index.
Removed the TODO comment from PartitionPruningRDD.
SPARK-673: Capture and re-throw Python exceptions

This patch alters the Python <-> executor protocol to pass on
exception data when they occur in user Python code.
Merge pull request #431 from mbautin/revert_default_profile

Remove activation of profiles by default
Some style cleanup
Small fix from last commit
Merge pull request #434 from pwendell/python-exceptions

SPARK-673: Capture and re-throw Python exceptions
Moved PruneDependency into PartitionPruningRDD.scala.
Fix stdout redirection in PySpark.
Merge pull request #435 from JoshRosen/pyspark_stdout_fix

Fix stdout redirection in PySpark.
Merge pull request #433 from rxin/master

Changed PartitionPruningRDD's split to make sure it returns the correct split index.
Stop BlockManagers metadataCleaner.
Do not launch JavaGateways on workers (SPARK-674).

The problem was that the gateway was being initialized whenever the
pyspark.context module was loaded.  The fix uses lazy initialization
that occurs only when SparkContext instances are actually constructed.

I also made the gateway and jvm variables private.

This change results in ~3-4x performance improvement when running the
PySpark unit tests.
Merge pull request #438 from JoshRosen/spark-674

Do not launch JavaGateways on workers (SPARK-674).
Use spark.local.dir for PySpark temp files (SPARK-580).
Merge pull request #439 from JoshRosen/spark-580

Use spark.local.dir for PySpark temp files (SPARK-580).
Merge pull request #437 from stephenh/cancelmetacleaner

Stop BlockManagers metadataCleaner.
Merge pull request #432 from stephenh/moreprivacy

Add more private declarations.