This includes the following changes:
- The "assembly" package now builds in Maven by default, and creates an
  assembly containing both hadoop-client and Spark, unlike the old
  BigTop distribution assembly that skipped hadoop-client
- There is now a bigtop-dist package to build the old BigTop assembly
- The repl-bin package is no longer built by default since the scripts
  don't reply on it; instead it can be enabled with -Prepl-bin
- Py4J is now included in the assembly/lib folder as a local Maven repo,
  so that the Maven package can link to it
- run-example now adds the original Spark classpath as well because the
  Maven examples assembly lists spark-core and such as provided
- The various Maven projects add a spark-yarn dependency correctly

- 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.