feature : zoom level

- fix : removed duplicated code between ImageResizer and CanvasUtils
  (disabledImageSmoothing utility method)
- added pskl.utils.UserAgent, basic user agent sniffer. I need it to sniff
  out IE10 for frame rendering (and it's not possible to feature detect
  here). Can check isChrome, isFirefox, isIE and get the version for each
  of them
- added resizeNearestNeighbour in ImageResizer. Readapted from piskel
  website, this allows us to 1 - resize without AA on IE10, and 2 - add a
  pixel gap to reenable the GRID
- finally : added back support for GRID !
- also extracted the 'zoomed out background color' as a constant in
  Constant.js

js/utils/CanvasUtils.js

@@ -32,6 +32,11 @@
       if (canvas) {
         canvas.getContext("2d").clearRect(0, 0, canvas.width, canvas.height);
       }
+    },
+
+    getImageDataFromCanvas : function (canvas) {
+      var sourceContext = canvas.getContext('2d');
+      return sourceContext.getImageData(0, 0, canvas.width, canvas.height).data;
     }
   };
 })();