Re-add support for reactions.

We now store emojis in a global cache and make things like adding
and removing reactions part of the stateful Message class.

discord/emoji.py

@@ -25,10 +25,13 @@ DEALINGS IN THE SOFTWARE.
 """
 
 import asyncio
+from collections import namedtuple
 
 from . import utils
 from .mixins import Hashable
 
+PartialEmoji = namedtuple('PartialEmoji', 'id name')
+
 class Emoji(Hashable):
     """Represents a custom emoji.