Create binary Spark distribution

This creates a new packaging target, a binary Spark distribution.  The existing scripts should continue to work with existing packaging (ie sbt package; run.....)

1) A new script, `make-distribution.sh`, creates a binary distro in dist/
2) The dist/ folder contains no sources, just a single assembly jar (for now), along with bin/ and conf/, and the run and spark-shell scripts.

The binary-only distribution makes deploys much easier, as it already contains Scala and all other dependencies packaged together.   Once you create the distribution, one could simply rsync the distro folder to the cluster, configure spark, and run the start scripts.

Caveats:
 - docs have not been updated yet
 - currently bagel, streaming, and the examples are excluded from the distro.