Mavenize and add unprotected root action

This plugin doesn’t work well with enabled access control in Jenkins. We need [UnprotectedRootAction](http://javadoc.jenkins-ci.org/hudson/model/UnprotectedRootAction.html) to disable authentication for `/gitlab` URL (as in the [GitHub Plugin](https://github.com/jenkinsci/github-plugin)) but this cannot be done from JRuby (Jenkins uses `instanceof` to determine if an Action is unprotected one). 

Workaround is quite simple, use ordinary Java class which implements the `UnprotectedRootAction` that just registers unprotected path. All other stuff is handled by JRuby code as before. However, I had to rewrite it to Maven project; I don’t know how to compile Java class and do other stuff in JRuby way.