Added class 'crn' to chron, read.chron, etc. so that plot.crn could be added as an S3 method.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@803 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Important note about backward compatibility

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@804 edb9625f-4e0d-4859-8d74-9fd3b1da38cb