Re-implement voice sending.

This is a complete redesign of the old voice code.

A list of major changes is as follows:

* The voice websocket will now automatically reconnect with
  exponential back-off just like the regular Client does.
* Removal of the stream player concept.
* Audio now gracefully pauses and resumes when a disconnect is found.
* Introduce a discord.AudioSource concept to abstract streams
* Flatten previous stream player functionality with the
  VoiceClient, e.g. player.stop() is now voice_client.stop()
* With the above re-coupling this means you no longer have to
  store players anywhere.
* The after function now requires a single parameter, the error,
  if any existed. This will typically be None.

A lot of this design is experimental.

discord/shard.py

@@ -110,6 +110,11 @@ class AutoShardedClient(Client):
         # the key is the shard_id
         self.shards = {}
 
+        def _get_websocket(guild_id):
+            i = (guild_id >> 22) % self.shard_count
+            return self.shards[i].ws
+
+        self.connection._get_websocket = _get_websocket
         self._still_sharding = True
 
     @asyncio.coroutine