Add BreitWigner function

Double_t TMath::BreitWigner(Double_t x, Double_t mean, Double_t gamma)
   // Calculate a Breit Wigner function with mean and gamma


git-svn-id: http://root.cern.ch/svn/root/trunk@4269 27541ba8-7e3a-0410-8455-c3a389f83636