SPARK-1066: Add developer scripts to repository.

These are some developer scripts I've been maintaining in a separate public repo. This patch adds them to the Spark repository so they can evolve here and are clearly accessible to all committers.

I may do some small additional clean-up in this PR, but wanted to put them here in case others want to review. There are a few types of scripts here:

1. A tool to merge pull requests.
2. A script for packaging releases.
3. A script for auditing release candidates.