Fix for bug #5305 - segfault while grouping on "very large" data + Updated NEWS.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1100 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Minor edit to NEWS entry

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1101 edb9625f-4e0d-4859-8d74-9fd3b1da38cb