Merge pull request #11 from arizzi/heppy_7_2_2_patch2

Thanks!
Adding cross section only on MC, and default to 1
replace 'from ROOT.reco import GenParticle' with 'GenParticle = ROOT.reco.GenParticle'
HeppyCore: time report functionality for heppy (run with -t or --timereport to enable)
Heppy: remove starts when storing all bits
Heppy: code to compute the leading track and its pt in a Jet
Heppy: make service looked for by the treeanalyzer configurable, default to 'outputfile'