further touch-ups for read.zoo and corresponding documentation

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@1008 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added read.zoo enhancements to NEWS

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@1009 edb9625f-4e0d-4859-8d74-9fd3b1da38cb