Use/pass DZI JSON directly instead of parsing DziTileSource manually

[Aseem Kishore]

Right now we parse the returned DZI JSON into a DziTileSource manually, because there used to be no other option. Now Seadragon Ajax supports taking DZI JSON directly (see update), so we should now let it.

This would not only simplify our code, it would also let Seadragon Ajax know the DZI URL (which gets lost if you instantiate a DziTileSource yourself), and knowing the DZI URL may be crucial to Silverlight support.