Add CMSSW_7_3_X IB.
Merge remote branch 'origin/IB/CMSSW_7_0_X/stable' into IB/CMSSW_7_0_X/stable
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable
Add CMSSW_7_3_X IB for slc6_amd64_gcc491
Revert to use stable root in CMSSW_7_2_THREADED_X
Merge pull request #1018 from ktf/revert-root-threaded

Revert to use stable root in CMSSW_7_2_THREADED_X
Merge remote branch 'origin/IB/CMSSW_7_0_X/stable' into IB/CMSSW_7_0_X/stable
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable
Add evntgen and photos++ to 7_1_X.

Superseeds #916.
Merge pull request #1019 from degano/photospp_evtgen_71X

Add evntgen and photos++ to 7_1_X.
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable
Merge remote branch 'origin/IB/CMSSW_7_2_X/stable' into IB/CMSSW_7_0_X/stable