Add back __repr__ that were deleted in the Asset redesign

The Asset PR (be227ebcf0) changed some namedtuple-deriving
classes to object-deriving classes, which meant that the free __repr__ provided by namedtuple
was removed

discord/invite.py

@@ -140,6 +140,10 @@ class PartialInviteGuild:
     def __str__(self):
         return self.name
 
+    def __repr__(self):
+        return '<{0.__class__.__name__} id={0.id} name={0.name!r} features={0.features} ' \
+               'description={0.description!r}>'.format(self)
+
     @property
     def created_at(self):
         """Returns the guild's creation time in UTC."""