Cleanup use of Script protocol.

I would think there would still be an error thrown here.  Will put together a test to determine for sure, but I'd think that the default GeoJSON parser used by the protocol would choke on the UTFGrid json.

lib/OpenLayers/Tile/UTFGrid.js

@@ -82,14 +82,12 @@ OpenLayers.Tile.UTFGrid = OpenLayers.Class(OpenLayers.Tile, {
             this.url = this.layer.getURL(this.bounds);
 
             if (this.layer.useJSONP) {
-                // Use JSONP method to avoid xbrowser polucy
-                var that = this;
-                var cback = function(resp) {
-                    that.json = resp.data;
-                };
+                // Use JSONP method to avoid xbrowser policy
                 var ols = new OpenLayers.Protocol.Script({
                     url: this.url, 
-                    callback: cback,
+                    callback: function(response) {
+                        this.json = response.data;
+                    },
                     scope: this
                 });
                 ols.read();