Add support for multiple file attachments.

This is a breaking change. No longer does Messageable.send have a
filename keyword argument, instead this is all handled through the
discord.File model. To upload many files you must specify a list
of discord.File objects.

discord/__init__.py

@@ -30,6 +30,7 @@ from .errors import *
 from .calls import CallMessage, GroupCall
 from .permissions import Permissions, PermissionOverwrite
 from .role import Role
+from .file import File
 from .colour import Color, Colour
 from .invite import Invite
 from .object import Object