Fixing JENKINS-16847 MultiJob Support

-Added a new Build Selector called MultiJobBuildSelector.
-If selected, this will make the Copy Artifact Plugin look
for an artifact from that job's build which was part of the
current MultiJobBuild.
-This selector will work properly if you configure it with
a MultiJobProject which has a PhaseJob that archives an
artifact that you want to retrieve as part of the MultiJob.
-This selector will also work if you configure it to Copy an
Artifact from another Phase Job which was part of the
current MultiJob Build.
-Added tests to ensure this functionality works as expected.
-Adds MultiJob Plugin as a dependency.
What do you mean not optional?  What specifically?