gitignore for JENKINS_HOME Jenkins settings (#1763)

* gitignore for JENKINS_HOME Jenkins settings

This allows an admin to use git to keep a backup of Jenkins settings
without tracking binary artifacts.  Useful for preserving settings during
plugin upgrades.

Note: secret.key is purposefully not tracked by git.  This should be
backed up separately because configs may contain secrets which were
encrypted using the secret.key.

See also:

* http://jenkins-ci.org/
* https://wiki.jenkins-ci.org/display/JENKINS/Administering+Jenkins

* Add a few entries to Jenkins gitignore