[JENKINS-24626] Added a selector "Downstream of"

[JENKINS-24626] (https://issues.jenkins-ci.org/browse/JENKINS-24626)

Added a selector "Downstream of", which allows to pick a build that is a downstream of a specific build.
As described in JENKINS-24626, parameters "Build selector for CopyArtifact" aren't compatible for upgrading Copyartifacts, and there's no workarounds for that.

![downstreamof](https://cloud.githubusercontent.com/assets/3115961/4347251/081c8294-4143-11e4-9fa1-23581643df99.png)