Add 5 new plugins to monitor Jenkins servers.

These are the the five new plugins:

jenkins_builds - to measure number of jenkins builds
jenkins_builds_results - to measure number of jenkins builds
jenkins_builds_running - to measure number of jenkins builds
which are currently running
jenkins_builds_results_summary - to measure results of all
jenkins builds
jenkins_jobs - to measure number of jenkins jobs

The plugins were written and are copyrighted by
Holger Levsen <holger@layer-acht.org> in 2012 and have been
released under the GNU Public Licence, version 2.
Merge remote branch 'pull/291' into devel

* pull/291:
  Add 5 new plugins to monitor Jenkins servers.
Adding a ref to HACKING.pod