Add script to automate config fetching

Summary:
This script does what I used to do by hand -- it fetches the latest
copies of all known configs from the Jenkins server. Then it's easy to
see a text diff of the changes and commit them.

Test Plan:
Ran ./fetch_jenkins_config.sh and verified that the new parts of the
server's configuration were copied locally. I then created other
changesets out of those changes.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2461