fix : reduce piskel model size

+ piskel deserialization is now clearly asynchronous
+ added utils.Deserializer (not a singleton though, more a builder/loader)
+ utils.Deserializer constructor expects a callback
+ when all layers are loaded and piskel is ready, the callback provided by
  the client is called with piskel as the first argument
- Deserializer doesn't fit in the utils package, which should be reserved
  to singletons : can move it to service as a PiskelLoaderService, and
  Deserializer could remain with only the purely static methods
- ImportController is realying on the Deserializer to build a Piskel but
  it shouldn't. Find a way to mutualize the code necessary to create a
  Piskel from an array of pskl.model.Frame
- still cleanup to do in app.js
- comments to add as well

js/controller/settings/ImportController.js

@@ -153,11 +153,15 @@
         var image = pskl.utils.ImageResizer.resize(this.importedImage_, w, h, smoothing);
         var frame = pskl.utils.FrameUtils.createFromImage(image);
 
-        var piskel = pskl.utils.Serializer.createPiskel([frame]);
-        pskl.app.piskelController.setPiskel(piskel);
-        pskl.app.animationController.setFPS(Constants.DEFAULT.FPS);
-
-        this.reset_();
+        // TODO : juliandescottes : here we only want to create a layer from an array of frames, and create a new piskel using this layer
+        // we shouldn't use the deserializer for this ... it's only working because it's falling back to the old implementation
+        // bad, very bad
+        var deserializer = new pskl.utils.Deserializer([frame], function (piskel) {
+          pskl.app.piskelController.setPiskel(piskel);
+          pskl.app.animationController.setFPS(Constants.DEFAULT.FPS);
+          this.reset_();
+        }.bind(this));
+        deserializer.deserialize();
       }
     }
   };