Support the Post Build Script plugin

Lets one execute a shell/groovy script or any builder AFTER the build
has complete.

https://wiki.jenkins-ci.org/display/JENKINS/PostBuildScript+Plugin

All existing builders should be supported since this patch call the
usual dispatcher.

Change-Id: Ide059ec9fd0e556f0c50ec992a6021eb5afb261c
Merge "Support the Post Build Script plugin"