Previous macro hadd.C renamed to hadd_old.C

Introduce a new macro fit2d.C to illustrate how to fit a 2-d histogram with
a distribution of the type y=ax+b with a straight line.


git-svn-id: http://root.cern.ch/svn/root/trunk@1607 27541ba8-7e3a-0410-8455-c3a389f83636