fixes #886 Add ColumnHider to header node when scrollbars are hidden.

When the scrollbars are hidden on OSX we want to add the ColumnHiderIcon node as a child of the header node. On a platform that has scrollbars we want to add the ColumnHiderIcon as a child of the headerScrollNode.

Also fixed a small IE11 bug which was causing us to fail to add the dgrid-rtl-swap class to the table. This would result in a small rendering bug for positioning the headerScrollNode properly.

Finally I added a test page for the column hider extension running on a RTL page.

Replaces https://github.com/SitePen/dgrid/pull/888
Fixes https://github.com/SitePen/dgrid/issues/886