Add tgz option to make-distribution.sh

This pull request builds on #662 by adding a tgz flag.  ie- `./make-distribution.sh tgz`  When tgz is given a spark-$VERSION-bin.tar.gz file will be generated from the dist/ directory.

Use cases for this functionality:

 * We have many spark environments and use chef to manage deploys and configs.  Using this method we can easily deploy/undeploy/re-deploy spark from a .tar.gz in a few seconds.
 * Should make generating binary releases easy