[Rivet] c++11 features in Rivet routines

[roman lysak]

   Dear Rivet authors,

we are in the process of validating one ATLAS analysis which using some 
C+11 features (e.g. brace-enclosed initializer of the list). I wonder 
what is your general view on this: do you accept such code already, or 
you would prefer to avoid such features for now? Specifically for this 
analysis, it should be easy to avoid c++11 features, but it would be 
good to know this also for the future analyses.

Thanks,
   Roman