[JENKINS-14154] Allow to access to other plugins

Recreation of #7.
[JENKINS-14154] (https://issues.jenkins-ci.org/browse/JENKINS-14154)

As groovy-postbuild uses it's own class loader for groovy scripts, scripts cannot access other plugins in Jenkins.
Switched to use `PluginManager.uberClassLoader`, which allows scripts to access other plugins.

It's safe as non-secured (non-whitelisted) methods will be forbidden by [Script Security plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Script+Security+Plugin).