Add Cover card and implement block

Assets/Battle/CardDatabase.cs

@@ -3,7 +3,8 @@ using System.Collections.Generic;
 using UnityEngine;
 
 public enum CardId {
-    CrashingPunch
+    CrashingPunch,
+    Cover
 }
 
 public class CardDatabase
@@ -13,6 +14,8 @@ public class CardDatabase
         {
             case CardId.CrashingPunch:
                 return new CardCrashingPunch();
+            case CardId.Cover:
+                return new CardCover();
             default:
                 throw new System.ArgumentException("CardId unknown", "id");
         }