Fixing JENKINS-16847 MultiJob Support

-Added a new Build Selector called MultiJobBuildSelector.
-If selected, this Selector allows you to input a job name.  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.