New message view added ('plain').

I like the included messages, but I wanted one more that was plain, with just a spinner. While you _do_ need to call setShowSpinner: despite the fact that this view only has a spinner, it does mean you can also have an even 'plainer' view that is just white if you don't.

Since the spinner lies in its superview's center, I've disabled adding a message for this view. I could always change it so you could do both, if that suits.

This view mimics a paging cell that ordinarily would require a lot of UITableViewController logic.