added vcov. argument to effect and Effect and by extension to allEffects.  seems to work

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/effects@170 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
typo fixed in a comment.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/effects@171 edb9625f-4e0d-4859-8d74-9fd3b1da38cb