Add Rivet 2.0 as separate package.

Note that it's disabled by default, enable it with "scram setup rivet2".
Merge pull request #441 from degano/rivet2_newpackage

Add Rivet 2.0 as separate package.
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable