add pileupjetid and tree code for pileupjetid validation

Probably worth updating the readme: if flashgg is already installed, cms-addpkg / cms-merge-topic crashes because "fatal: destination path '(...)/CMSSW_7_0_7_patch1/src' already exists and is not an empty directory.", so this should be run before checking out flashgg (until this is merged in some future release ?)

Cheers,
Olivier