adding the addLines function to add event lines

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@850 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
s/plot2_xts/plot

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@851 edb9625f-4e0d-4859-8d74-9fd3b1da38cb