Implements FR #2405. Extensive tests added.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1254 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Implements FR #5465. Test added + News updated.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1255 edb9625f-4e0d-4859-8d74-9fd3b1da38cb