Changed the default nlags to NULL, modified structure/text of the manuals, fixed a bug in subperiod.lmAMM to handle xts. Made classes of return values consistent.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@282 edb9625f-4e0d-4859-8d74-9fd3b1da38cb