Merge the changes from David in the dev branch (up to rev 31490) to return the TFitResultPtr from the Fit methods of 
TGraph, TGraph2D, TMultiGraph, THnSparse


git-svn-id: http://root.cern.ch/svn/root/trunk@31491 27541ba8-7e3a-0410-8455-c3a389f83636