+ x[y] now starts from the middle of y
+ rolltolast now issues warning that it was deprecated a year ago, updated tests to use rollends, and NEWS.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1147 edb9625f-4e0d-4859-8d74-9fd3b1da38cb