add tile merging

js/tile.js

@@ -4,8 +4,14 @@ function Tile(position, value) {
   this.value            = value || 2;
 
   this.previousPosition = null;
+  this.mergedFrom       = null; // Tracks tiles that merged together
 }
 
 Tile.prototype.savePosition = function () {
   this.previousPosition = { x: this.x, y: this.y };
 };
+
+Tile.prototype.updatePosition = function (position) {
+  this.x = position.x;
+  this.y = position.y;
+};