Issue 24 : Layers

!! NOT STABLE !!
Initial implementation. No UI update yet.
Check js/model/Piskel.js and js/model/Layer.js for an overview of the new
API.

Piskels can be saved on the existing service.

Previous piskels cannot be loaded. This should be fixed soon.

js/utils/FrameUtils.js

@@ -0,0 +1,22 @@
+(function () {
+  var ns = $.namespace('pskl.utils');
+
+  ns.FrameUtils = {
+    merge : function (frames) {
+      var merged = frames[0].clone();
+      var w = merged.getWidth(), h = merged.getHeight();
+      for (var i = 1 ; i < frames.length ; i++) {
+        pskl.utils.FrameUtils.mergeFrames_(merged, frames[i]);
+      }
+      return merged;
+    },
+
+    mergeFrames_ : function (frameA, frameB) {
+      frameB.forEachPixel(function (p, col, row) {
+        if (p != Constants.TRANSPARENT_COLOR) {
+          frameA.setPixel(col, row, p);
+        }
+      });
+    }
+  };
+})();