SPARK-2416: Allow richer reporting of unit test results

The built-in Jenkins integration is pretty bad. It's very confusing to users whether tests have passed or failed and we can't easily customize the message.

With some small scripting around the Github API we can do much better than this.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #1340 from pwendell/better-qa-messages and squashes the following commits:

fd6077d [Patrick Wendell] Better automation for unit tests.