Added persistence of game state to localstorage after each move

Added rebuilding of game state from localstoage on page load
Added New Game button to allow game restarts now that refreshes resume the game

js/grid.js

@@ -1,20 +1,32 @@
-function Grid(size) {
+function Grid(size, previousCellState) {
   this.size = size;
-
-  this.cells = [];
-
-  this.build();
+  this.cells = previousCellState ? this.buildFromPreviousState(previousCellState) : this.buildNew();
 }
 
 // Build a grid of the specified size
-Grid.prototype.build = function () {
+Grid.prototype.buildNew = function () {
+  var cells = [];
   for (var x = 0; x < this.size; x++) {
-    var row = this.cells[x] = [];
+    var row = cells[x] = [];
 
     for (var y = 0; y < this.size; y++) {
       row.push(null);
     }
   }
+  return cells;
+};
+
+Grid.prototype.buildFromPreviousState = function (state) {
+    var cells = [];
+    for (var x = 0; x < this.size; x++) {
+        var row = cells[x] = [];
+
+        for (var y = 0; y < this.size; y++) {
+            var tileState = state[x][y];
+            row.push(tileState ? new Tile(tileState.position, tileState.value) : null);
+        }
+    }
+    return cells;
 };
 
 // Find the first available random position
@@ -82,3 +94,19 @@ Grid.prototype.withinBounds = function (position) {
   return position.x >= 0 && position.x < this.size &&
          position.y >= 0 && position.y < this.size;
 };
+
+Grid.prototype.gridState = function () {
+  var cellState = [];
+  for (var x = 0; x < this.size; x++) {
+    var row = cellState[x] = [];
+
+    for (var y = 0; y < this.size; y++) {
+        row.push(this.cells[x][y] ? this.cells[x][y].tileState() : null);
+    }
+  }
+
+  return {
+      size: this.size,
+      cells: cellState
+  }
+};