started fiddling with the plot.sts.time function for more flexible formatting of the x-axis using the xaxis.labelFreq and xaxis.tickFreq arguments. ToDo: Document and possible simplify

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@821 edb9625f-4e0d-4859-8d74-9fd3b1da38cb