Added performance.now polyfill.

Author: photonstorm

src/polyfills.js

@@ -154,3 +154,31 @@ if (!window.console)
     window.console.log = window.console.assert = function(){};
     window.console.warn = window.console.assert = function(){};
 }
+
+/**
+ * performance.now
+ */
+(function(){
+
+  if ("performance" in window == false) {
+      window.performance = {};
+  }
+  
+  Date.now = (Date.now || function () {  // thanks IE8
+      return new Date().getTime();
+  });
+
+  if ("now" in window.performance == false)
+  {
+    var nowOffset = Date.now();
+    
+    if (performance.timing && performance.timing.navigationStart){
+      nowOffset = performance.timing.navigationStart
+    }
+
+    window.performance.now = function now(){
+      return Date.now() - nowOffset;
+    }
+  }
+
+})();