Heppy updates: time report and bugfixes

Hi Colin,

A few small updates:
   * adding an optional time report functionality
   * fixing the adding of the cross section, as discussed in #10 
   * change the `from ROOT.reco import GenParticle` line (from #11) with an equivalent syntax which works also in our setup while the original didn't (maybe it's the difference between running with `heppy` or from the python cfg directly?)

@arizzi