Add webhook support.

Allows for usage of either `requests` and `aiohttp` when used in
"Standalone" mode.

Fixes #704

discord/__init__.py

@@ -41,6 +41,7 @@ from collections import namedtuple
 from .embeds import Embed
 from .shard import AutoShardedClient
 from .player import *
+from .webhook import *
 from .voice_client import VoiceClient
 from .audit_logs import AuditLogChanges, AuditLogEntry, AuditLogDiff