[FIXED JENKINS-24626] Added a new build selector "Downstream of"
[JENKINS-24626] Added permission check, help texts for DownstreamBuildSelector.
[JENKINS-24626] Added tests for DownstreamBuildSelector.
[JENKINS-24626] Add notices for compatibility of BuildSelectorParameter.
Merge pull request #44 from ikedam/feature/JENKINS-24626_DownstreamOf

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