Implement publisher.checkstyle

Add publisher for Checkstyle Plugin[1] (based on code for the
Violations Plugin publisher)

[1] https://wiki.jenkins-ci.org/display/JENKINS/Checkstyle+Plugin

Change-Id: Ie622ad8600720081072f288e72dd56e65505380a
Reviewed-on: https://review.openstack.org/21066
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins