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/service/LocalStorageService.js

@@ -45,8 +45,10 @@
    */
   ns.LocalStorageService.prototype.restoreFromLocalStorage_ = function() {
     var framesheet = JSON.parse(window.localStorage.snapShot);
-    var piskel = pskl.utils.Serializer.createPiskel(framesheet);
-    pskl.app.piskelController.setPiskel(piskel);
+    var deserializer = new pskl.utils.Deserializer(framesheet, function (piskel) {
+      pskl.app.piskelController.setPiskel(piskel);
+    });
+    deserializer.deserialize();
   };
 
   /**