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);
 

examples/utfgrid_twogrids.html

@@ -48,11 +48,13 @@
 
         var world_utfgrid = new OpenLayers.Layer.UTFGrid( 
             "World Population",
-            "./utfgrid/world_utfgrid/${z}/${x}/${y}.json"
+            "./utfgrid/world_utfgrid/${z}/${x}/${y}.json",
+            {utfgridResolution: 4} // default is 2
         );
         var bio_utfgrid = new OpenLayers.Layer.UTFGrid( 
             "World Bioregions",
-            "./utfgrid/bio_utfgrid/${z}/${x}/${y}.json"
+            "./utfgrid/bio_utfgrid/${z}/${x}/${y}.json",
+            {utfgridResolution: 4} // default is 2
         );
         map.addLayers([bio_utfgrid,world_utfgrid]);
 

lib/OpenLayers/Layer/UTFGrid.js

@@ -163,8 +163,8 @@ OpenLayers.Layer.UTFGrid = OpenLayers.Class(OpenLayers.Layer.Grid, {
      * APIProperty: utfgridResolution
      * {Number}
      * Ratio of the pixel width to the width of a UTFGrid data point.  If an 
-     *     entry in the grid represents a 2x2 block of pixels, the 
-     *     utfgridResolution would be 2.  Default is 4 (specified in 
+     *     entry in the grid represents a 4x4 block of pixels, the 
+     *     utfgridResolution would be 4.  Default is 2 (specified in 
      *     <OpenLayers.Tile.UTFGrid>).
      */
 

lib/OpenLayers/Tile/UTFGrid.js

@@ -36,10 +36,10 @@ OpenLayers.Tile.UTFGrid = OpenLayers.Class(OpenLayers.Tile, {
      * Property: utfgridResolution
      * {Number}
      * Ratio of the pixel width to the width of a UTFGrid data point.  If an 
-     *     entry in the grid represents a 2x2 block of pixels, the 
-     *     utfgridResolution would be 2.  Default is 4.
+     *     entry in the grid represents a 4x4 block of pixels, the 
+     *     utfgridResolution would be 4.  Default is 2.
      */
-    utfgridResolution: 4,
+    utfgridResolution: 2,
     
     /** 
      * Property: json

tests/Tile/UTFGrid.html

@@ -8,7 +8,7 @@
         layer = new OpenLayers.Layer.UTFGrid(
             null, 
             "../data/utfgrid/world_utfgrid/${z}/${x}/${y}.json",
-            {isBaseLayer: true}
+            {isBaseLayer: true, utfgridResolution: 4}
         );
         map = new OpenLayers.Map({
             div: "map",