Modified the eventstudy interface, changed the man pages and test cases.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@361 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Continue to next iteration if a NULL is found, don't return NULL. Return NULL only if the best lag could not be found.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@362 edb9625f-4e0d-4859-8d74-9fd3b1da38cb