Add created_at property to PartialEmoji

Also fixed docstring for PartialEmoji.url

discord/partial_emoji.py

@@ -136,9 +136,20 @@ class PartialEmoji(_EmojiTag):
             return self.name
         return '%s:%s' % (self.name, self.id)
 
+    @property
+    def created_at(self):
+        """Optional[:class:`datetime.datetime`]: Returns the emoji's creation time in UTC, or None if Unicode emoji.
+
+        .. versionadded:: 1.6
+        """
+        if self.is_unicode_emoji():
+            return None
+
+        return utils.snowflake_time(self.id)
+
     @property
     def url(self):
-        """:class:`Asset`:Returns an asset of the emoji, if it is custom."""
+        """:class:`Asset`: Returns an asset of the emoji, if it is custom."""
         if self.is_unicode_emoji():
             return Asset(self._state)