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.

docs/api.rst

@@ -827,6 +827,12 @@ Embed
 .. autoclass:: Embed
     :members:
 
+File
+~~~~~
+
+.. autoclass:: File
+    :members:
+
 CallMessage
 ~~~~~~~~~~~~