[JENKINS-40766] add pipeline support

[estarter]

Pipeline support is added to release plugin. Release can be triggered from pipeline as following: m2release job: 'jobname'

I was using pipeline-build-step-plugin as a reference as well as release-plugin.

I've done few modifications in order to be able to start developing on the plugin (currently compilation is failing on master brunch):

1. fixed javadoc errors
2. junit tests updated to use @Rules instead of inheriting HudsonTestCase
3. missing header <?jelly escape-by-default='true'?> was added to *.jelly files