- Don't attempt to run worker processes with ipython (that can cause
  some crashes as ipython prints things to standard out)
- Allow passing some IPYTHON_OPTS to launch things like the notebook

Expand the PySpark programming guide.

- Bundle Py4J binaries, since it's hard to install
- Uses Spark's `run` script to launch the Py4J
  gateway, inheriting the settings in spark-env.sh

With these changes, (hopefully) nothing more than
running `sbt/sbt package` will be necessary to run
PySpark.