Comments and NEWS tidy up only, no code changes.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1167 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Spelling error in NEWS

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1168 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Corrected timing for dcast.data.table in NEWS

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1169 edb9625f-4e0d-4859-8d74-9fd3b1da38cb