[FIXED JENKINS-23127] Allow loading classes from plugins.
Pending a rework of #538, the least we can do is show users the wiki page with suggestions.
Cf.: http://stackoverflow.com/questions/8109607/headless-continuous-integration-with-jenkins
Merge branch 'fix' of github.com:KostyaSha/jenkins
[JENKINS-23127] Noting merge of #1244.