Implement option to use the old branch style.

By default Event uses the new branch style.
To use the old branch style, specify teh split value negative, eg:
  Event 200 0 -1 1
generates 200 events in the old split mode.


git-svn-id: http://root.cern.ch/svn/root/trunk@2070 27541ba8-7e3a-0410-8455-c3a389f83636