[SPARK-5808] [build] Package pyspark files in sbt assembly.

This turned out to be more complicated than I wanted because the
layout of python/ doesn't really follow the usual maven conventions.
So some extra code is needed to copy just the right things.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5461 from vanzin/SPARK-5808 and squashes the following commits:

7153dac [Marcelo Vanzin] Only try to create resource dir if it doesn't already exist.
ee90e84 [Marcelo Vanzin] [SPARK-5808] [build] Package pyspark files in sbt assembly.