Add <User/Guild/Role/Channel>.createdAt (#620)

* Add Role.createdAt

* Add Channel.createdAt

* Add Guild.createdAt

* Add User.createdAt

* Change createdAt to Unix Timestamp

* Add createdAt to Emojis

* Change createdAt to Unix Timestamp

src/structures/Guild.js

@@ -332,6 +332,15 @@ class Guild {
     }
   }
 
+  /**
+   * The unix timestamp the guild was created
+   * @readonly
+   * @type {Date}
+   */
+  get createdAt() {
+    return new Date((+this.id / 4194304) + 1420070400000).getTime();
+  }
+
   /**
    * The date at which the logged-in client joined the guild.
    * @type {Date}