Making the default UTFGrid resolution 2.

According to the latest [spec](https://github.com/mapbox/utfgrid-spec/blob/master/1.2/utfgrid.md): "By default, UTFGrid operates on a 2x2 grid."

examples/utfgrid.html

@@ -127,7 +127,8 @@
 
         var grid_layer = new OpenLayers.Layer.UTFGrid( 
             'Invisible UTFGrid Layer', 
-            "./utfgrid/world_utfgrid/${z}/${x}/${y}.json"
+            "./utfgrid/world_utfgrid/${z}/${x}/${y}.json",
+            {utfgridResolution: 4} // default is 2
         );
         map.addLayer(grid_layer);