Now ADD_FILES uses a comma as file name separator.

Also, adds a line in doc explaining how to use.

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.

Suggested by / based on code from @MLnick