Add group by, fix order by
Get group by and order by to work with XM.XdrupleCommerceProduct
Fix order by by wrapping unions in subquery table.
Add etag support.
Fix etag error. Expose _fetch() so it be used in xdruple ext.
Use XM.ItemSitePrivate model for this.
Merge pull request #1489 from bendiy/exclusive_items

XM.ItemSiteListItem._fetch Add group by, fix order by