Merge firm returns and market returns before calling lm, removing it had caused a bug where lm wouldn't return a proper zoo object in the residuals series.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@397 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Return model residuals as an attribute to the final object.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@398 edb9625f-4e0d-4859-8d74-9fd3b1da38cb