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/Emoji.js

@@ -43,6 +43,15 @@ class Emoji {
     this.managed = data.managed;
   }
 
+  /**
+   * The unix timestamp the emoji was created
+   * @readonly
+   * @type {Date}
+   */
+  get createdAt() {
+    return new Date((+this.id / 4194304) + 1420070400000).getTime();
+  }
+
   /**
    * A collection of roles this emoji is active for (empty if all), mapped by role ID.
    * @type {Collection<string, Role>}