changed plotting to have a xaxis.dateFormat argument, which allows for very flexible plotting of the xaxis. Note: line argument is currently 0. Needs to be 1 to correspond to old function. No documentation and R cmd check stuff done yet. As a part made formatDate work for windows even for %G and %V arguments and optimized for speed

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@824 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added testthat for the new formatDate func

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@825 edb9625f-4e0d-4859-8d74-9fd3b1da38cb