Fix : add backup service and make undo safer

src/js/model/Layer.js

@@ -96,4 +96,10 @@
   ns.Layer.prototype.length = function () {
     return this.frames.length;
   };
+
+  ns.Layer.prototype.getHash = function () {
+    return this.frames.map(function (frame) {
+      return frame.getHash();
+    }).join('-');
+  };
 })();

src/js/model/Piskel.js

@@ -109,4 +109,10 @@
     var appEngineEditorHeader = $('.piskel-name').html(this.descriptor.name);
   };
 
+  ns.Piskel.prototype.getHash = function () {
+    return this.layers.map(function (layer) {
+      return layer.getHash();
+    }).join('-');
+  };
+
 })();