fix : reduce piskel model size

+ moved Serializer and Deserializer to utils.serialization package
+ put all backward code in utils.serialization.backward
+ added static method on Deserializer to make its usage similar to other
  utils in the package
- still not happy with the names used in Deserializer classes
  (deserializer.deserialize ...)

js/utils/serialization/Serializer.js

@@ -0,0 +1,31 @@
+(function () {
+  var ns = $.namespace('pskl.utils');
+
+  ns.Serializer = {
+    serializePiskel : function (piskel) {
+      var serializedLayers = piskel.getLayers().map(function (l) {
+        return pskl.utils.Serializer.serializeLayer(l);
+      });
+      return JSON.stringify({
+        modelVersion : Constants.MODEL_VERSION,
+        piskel : {
+          height : piskel.getHeight(),
+          width : piskel.getWidth(),
+          layers : serializedLayers
+        }
+      });
+    },
+
+    serializeLayer : function (layer) {
+      var frames = layer.getFrames();
+      var renderer = new pskl.rendering.FramesheetRenderer(frames);
+      var base64PNG = renderer.renderAsCanvas().toDataURL();
+
+      return JSON.stringify({
+        name : layer.getName(),
+        base64PNG : base64PNG,
+        frameCount : frames.length
+      });
+    }
+  };
+})();