Add remove() to Reaction

Added a coro, `remove()` which takes in a sole parameter, `member`.
This new coro will remove the reaction by the provided member from the reactions message.

`message.remove_reaction(reaction, member)` was not removed as to not introduce breaking changes.

discord/reaction.py

@@ -93,6 +93,34 @@ class Reaction:
     def __repr__(self):
         return '<Reaction emoji={0.emoji!r} me={0.me} count={0.count}>'.format(self)
 
+    async def remove(self, user):
+        """|coro|
+
+        Remove the reaction by the provided :class:`User` from the message.
+
+        If the reaction is not your own (i.e. ``user`` parameter is not you) then
+        the :attr:`discord.permissions.Permissions.manage_messages` permission is needed.
+
+        The ``user`` parameter must represent a user or member and meet
+        the :class:`abc.Snowflake` abc.
+
+        Parameters
+        -----------
+        user: :class:`abc.Snowflake`
+             The user or member from which to remove the reaction.
+
+        Raises
+        -------
+        HTTPException
+            Removing the reaction failed.
+        Forbidden
+            You do not have the proper permissions to remove the reaction.
+        NotFound
+            The user you specified, or the reaction's message was not found.
+        """
+
+        await self.message.remove_reaction(self.emoji, user)
+
     def users(self, limit=None, after=None):
         """Returns an :class:`AsyncIterator` representing the users that have reacted to the message.