Implemented extension point to the compare functionality.

As discussed with Jesse at the copenhagen code camp this is an extension point that allows other plugins to control whether a build should be triggered or not. Usage example can be seen here https://github.com/pretested-integration/pretested-integration-mercurial/blob/master/src/main/java/org/jenkinsci/plugins/pretestedintegrationmercurial/MercurialComparator.java