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