Qg tagger update nov2014

Update the QGTagger.cc file with:
- thread-safe implementation of jet corrections on the fly (based on https://twiki.cern.ch/twiki/bin/view/CMSPublic/FWMultithreadedConsumesJetCorrectorMigration)
- introducing pt > 1 GeV cut on neutral particles for axis2 and ptD calculation (improves slightly the stability versus pile-up)
- make the dz and dsigma cuts optional ("useQualityCuts" boolean, false by default): the impact of these cuts on the performance of the tagger seems to be very small on MC. Without them we can easily apply the tagger also on miniAOD, and therefore we switched them off at the moment. Though the possibility to switch them back on later is kept, because there is a possibiltiy they give better data/MC agreement.
I removed the sqrt. At the moment the d0_sigma will not be used, I just keep the possiblity (with the switch "useQualityCuts = cms.bool(True)") to re-introduce them in the future if needed.