Automatically set Yarn env vars in PySpark (SPARK-1030).
WIP towards PySpark on YARN:

- Remove reliance on SPARK_HOME on the workers.  Only the driver
  should know about SPARK_HOME.  On the workers, we ensure that the
  PySpark Python libraries are added to the PYTHONPATH.

- Add a Makefile for generating a "fat zip" that contains PySpark's
  Python dependencies.  This is a bit of a hack and I'd be open to
  better packaging tools, but this doesn't require any extra Python
  libraries.  This use case doesn't seem to be well-addressed by the
  existing Python packaging tools: there are plenty of tools to package
  complete Python environments (such as pyinstaller and virtualenv) or
  to bundle *individual* libraries (e.g. distutils), but few to generate
  portable fat zips or eggs.

This hasn't been tested with YARN and may not actually compile.