Added class changes to the read.rwl family so that classes are now c("rwl","data.frame"). I'll stop now because I know Mikko will want to fixed everything I've done wrong!

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@751 edb9625f-4e0d-4859-8d74-9fd3b1da38cb