Adding BoostedJetMerger to PhysicsTools/PatUtils

Adding BoostedJetMerger plugin to PhysicsTools/PatUtils. Developed for jet substructure studies using PAT and originally added to TopQuarkAnalysis/TopPairBSM (http://cvs.web.cern.ch/cvs/cgi-bin/viewcvs.cgi/CMSSW/TopQuarkAnalysis/TopPairBSM/src/?pathrev=V04-02-09) but never included in any official release.