Implement cardBacks.

js/Card.js

@@ -11,12 +11,17 @@ class Card {
         this.symbol = symbol;
         this.points = points;
         this.assetUrl = this.createAssetPath(assetBaseUrl);
+        this.backAssetUrl = this.createBackAssetPath(assetBaseUrl);
     }
 
     createAssetPath(assetBaseUrl) {
         return assetBaseUrl + this.color + this.symbol + '.png'
     }
 
+    createBackAssetPath(assetBaseUrl) {
+        return assetBaseUrl + 'back.png'
+    }
+
     isAnAce() {
         return this.symbol === 'a';
     }