JENKINS-18737 Whitelist Branches

I have modified the GHPRB to allow for filtering of pull requests by specified destination (target) branches. Modified ghprb.java, GhprbTrigger.java and GhprbPullRequest.java. Also modified config.java and added help-branchList.html for the help message. Tests were also updated accordingly to test the newly added code.  This implements the feature that was requested by JENKINS-18737.