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.

docs/api.rst

@@ -40,6 +40,21 @@ Client
 .. autoclass:: AutoShardedClient
     :members:
 
+Voice
+------
+
+.. autoclass:: VoiceClient
+    :members:
+
+.. autoclass:: AudioSource
+    :members:
+
+.. autoclass:: PCMAudio
+    :members:
+
+.. autoclass:: FFmpegPCMAudio
+    :members:
+
 Opus Library
 ~~~~~~~~~~~~~