Fixed test case to use Date class in eventlist.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@373 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added more description on date-time class of event list and timeseries object.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@374 edb9625f-4e0d-4859-8d74-9fd3b1da38cb