properly register all rollsum methods

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@1013 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
reverted screens change in ggplot2 functions because it still needs to be improved

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@1014 edb9625f-4e0d-4859-8d74-9fd3b1da38cb